[layout] Fix tools:context references to MainActivity
[canvas] Get rid of the init() method in AbstractUIElement
[widget] Proper TweetLayoutView implementation for onMeasure/onLayout
[widget] Use layout_weight for action icons in TweetCompositeView
[widget] Implement shouldDelayChildPressedState() in custom ViewGroups
[canvas] Change ElementGroup's API to add UIElements
[canvas] Support LayoutParams and inflation with UIElements