Changes:
* Add DataRecoveryView
* Split SubObjectsIO into an interface and SubObjectsFromSprite
* Add a "setId" parameter to some doc::read_* functions (we need this
functionality from session recovery)
* Add LinkLabel::onClick()
* Use Widget instead of ListItem in ListBox API
* Add TextRule::padding() and use it to calculate the preferred size
for a skin::Style's rules
* Add widgets/home_view.xml
* Add possibility to specify style="..." and border="..." in XML widgets
* Add SkinStyleProperty
* Remove Label::get/setTextColor() (replaced with styles)
* Add <image> widget in WidgetLoader
* Paint labels, link labels, and views with styles