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

* Add a button to the modal example to hide modal

* Render button only for iOS

Until now, buttons would use the same font family used by the title,
this commit introduces navBarButtonFontFamily property which allows to set
a different font family for title and buttons.

fixes #2038

* Support subtitle FontFamily on size on Android

Font family can be either a .ttf/.otf asset or one of the default font families:

* sans-serif (regular)
* sans-serif-light
* sans-serif-condensed
* sans-serif-thin
* sans-serif-medium

* update docs

While working I got a situation in which I wanted to open the side drawer on a button click. But I was not able to that. So I made some changes and now I am able to achieve the required functionality.

This prevented custom react views in navBar from being displayed if AppStyle was used.

* Fix style

* Fix resolving logic

* Fix styling

Fixes #1938 Closes #1900

This commit fixes two issues found in #1838. Fixes #1924
1. Keeps calling the deprecated reactInstanceManager methods when host is cleared
    to not introduce undesired behavior when clearHostOnActivityDestroy return true.
2. call onHostDestroy and onHostPause only if react context exists or is being created.

* remove unused code from tabStyle

* improve dummy screen ui

* set list screen translucent navigation bar

* make RCCToolBarView to composite the UIRootView inside

* add list screen for debug behaviours

* add dummy screen

* ui changes in ListScreen

* add push list screen function without add it to the button to the screen

* add and comment push list screen row

This only happens when you are using react-native@0.48.3, and your XCode is in version 9.0

* Update the comment for tabsStyle in Android

* for -> on

* Update top-level-api.md

Fixes #1844

This commit adds support for fade and slide-horizontal animations for modals and a few other animation related changes:

* Support defining animationType when calling dismissModal
* Support fade and slide horizontal animations for modals
* Slightly reworked current screen animations timings
* Fixed slide-horizontal x offset and removed alpha

This is a breaking change and has to be opt-in. To prevent host.clear from being called,
Override `clearHostOnActivityDestroy` in MainApplication and return false:

```java
    @Override
    public boolean clearHostOnActivityDestroy() {
        return false;
    }
```

If host isn't cleared, the next time the app is opened react context might still be initialized.
In this case we emit appLaunched event which has to be handled in Js:

```js
  Promise.resolve(Navigation.isAppLaunched())
      .then((appLaunched) => {
        if (appLaunched) {
          startApp();
        }
        new NativeEventsReceiver().appLaunched(() => {
          startApp();
        });
      })
```

This converts from using a deprecated attribute.

* add disabledButtonColor in order to set navigation button disabled color

* add disabledButtonColor to the docs

Fixes #1007