TIMOB-13392: Android scrollable tabs

Android tab group can now by default be navigated by swipes (e.g. see Play Store app). The functionality can be toggled with the new `TabGroup.swipeable` property. In addition, tab navigation can be disabled (or re-enabled) by calling `disableTabNavigation(boolean)`. Plus added tab `selected` and `unselected` events (currently these events exist in Android only. In iOS the tab focus/blur events do the same - I suggest they be renamed to selected/unselected). Note that all additional events are fired exactly as before.
@pingwang2011 well, not sure there is much utility for using a setter/getter for a boolean which is internal to the implementation..... e.g. no real difference between `tabgroup.swipeable = true` and using a setter.

In any case, please let me know when you are finished with comments since I don't want to make fix after fix.