* Add converter for UIBarButtonSystemItem

* Add support for UIBarButtonSystemItem-styled buttons

* Add documentation about systemItem

* setTabButton -> has now the property 'label' to change the label of the TabButton

* Add to documentation

* [BugFix] use i instead of 0 to check all TabItems

Add the preview and pop feature

To use the feature you simply call `navigator.push()` but with additional parameter `previewViewID` which is a React Node.

```js
<TouchableOpacity
  onPressIn={() => this.props.navigator.push({
    screen: 'screenid',
    previewView: this.viewRef,
    previewActions: [{
      title: 'Foo',
      style: 'selected', // none, selected, destructive
      actions: [{ title: 'Bar' }],
    }],
 })
/>
```

This commit saves navBar component props in propsRegistry and restores the saved props
when the component is mounted, allowing us to pass unserializable props to custom navBar component.

This fixes a bug where navigator-level styles could override styles on a specific button.

This pull request allows you to disable the open gesture only in one side of the drawer.

```
Navigation.startSingleScreenApp({
      screen: initialScreen,
      drawer: {
        left: {
          screen: DRAWER_SCREEN,
          disableOpenGesture: false,
        },
        right: {
          screen: SIDEBAR_FILTER_SCREEN,
          disableOpenGesture: true,
        },
      },
    });
```

If we pass disableOpenGesture to the drawer root object, like this: 

```
Navigation.startSingleScreenApp({
      screen: initialScreen,
      drawer: {
        left: {
          screen: DRAWER_SCREEN,
          disableOpenGesture: false,
        },
        right: {
          screen: SIDEBAR_FILTER_SCREEN,
          disableOpenGesture: true,
        },
        disableOpenGesture: true <======
      },
    });
```

Then the two drawer positions would be gesture disabled and the specific disableOpenGesture would be ignored. This way we don`t introduce breaking changes.

This reverts commit 9f2f15bc4f.

* Support iOS 11 prefersLargeTitles #1643

* better and more simple version checking and setting

This commit applies the save props mechanism used when pushing screen
to navigator buttons as well, enabling us to pass unserializable props to buttons.

* throwing an error instead of a string

* return a callback to remove the register handler

Fix props loading from PropsRegistry in custom buttons

This reverts commit e69f3801b9.

This fix allows to pass unserializable props to custom buttons, such as navigator or functions.

fixes #2166

This commit adds ability to register multiple navigator event listeners:
navigator.addOnNavigatorEvent(func)
navigator.removeOnNavigatorEvent(func)

Event listeners are removed when the screen the navigator is bound to
is unmounted, calling `removeOnNavigatorEvent` isn't mandatory.

As this api conflicts with the current api, addOnNavigatorEvent can't be used
in a screen where setOnNavigatorEvent was used before.

On some devices, when returning to the app from background, some launchers
open the SplashActivity over the NavigationActivity. In this case, simply finish the SplashActivity
and return the the preview NavigationActivity.

This is potentially a breaking change as calling navigator.switchToTab()
won't do anything now if the current tab is already selected.

closes #2143

No major changes, just tweaked values a little bit.

* Fab animation duration changed to 120 (previously 200)
* Fab show animation: Scales from 0.6 to 1 (previously 0)
* Fab hide animation: Scales from 1 to 0.6 (previously 0)
* Removed alpha delay from screen animation
* TranslationY value changed from 8% to 5%

* Support showing modals with the default screen push/pop animation

* Set navigation bar color before modal is displayed

* Implement screenAnimationType property

controls animation type of all screens including modals.

added some elaborations to make it easier for developers with no ios experience.

Since status bar style was applied when the screen was created, this caused a noticeable
artifact in status bar color.

When toggleTabs was called, the visibility state of the BottomTabs
was not persisted. There for, when pushing a screen and popping
the BottomTabs visibility state was set to the value of bottomTabsHidden
from navigatorStyle.

This commit continues the work done in #2057.
When hardware back button is pressed, we handle it accordingly:

1. If screen was pushed with `overrideBackPress: true`, we emit `backPress` event
2. Let `BackHandler` handle the event
3. If the event wasn't handled by `BackHandler`, pop the stack
4. let the system handle the event

This reverts commit 3dd185ecad.

Perform all stack related commands in performOnStack which
doesn't crash when requested stack is no found