Change bottom tab bar button image at runtime

This commit adds support for the following style properties:
titleBarTitleFontFamily - string, font name in assets
titleBarTitleTextCentered - boolean, default is false

* Add font family support on bottom tabs

* import missing classes

* import missing class

* update document

This change means that in a tab based app, when pushing a screen
which did not declare `tabBarBackgroundColor` - the BottomTabs color
will persist from the previous screen and not be set to the default
color which is white. This eliminates the unwanted BottomTabs
background color change in this specific use case.

closes #409

fixes #253

works best with drawUnderTabBar

Since we don't fully support hideOnScroll property yet, draw all views
above BottomTabs