A container that manages appBar, bottomBar, and a scrollable child Widget.
AppBar will be collapsed/expanded accordding to scrolling offset.
All the bars are customizable widgets.
The scrollable child must be one of the following widget:
* JkAppBarListView (a clone of official ListView but support bars collapsed/expanded)
* JkAppBarGridView (a clone of official GridView but support bars collapsed/expanded)
* JkAppBarSingleChildScrollView (a clone of official SingleChildScrollView but support bars collapsed/expanded)
Any other scrollable widget won't make app bars collapsed/expanded during scrolling.