Add css library

This is a base library that will be used by SkinTheme to draw parts
with CSS styles (or something similar).
Minor refactor in skin properties

- Add get_skin_property(ui::Widget*) function to avoid duplicating code
  to create a SkinProperty.
- A SkinSliderProperty is not a SkinProperty: now it has its own name.
Merge branch 'dev' into timeline
Add support for vertical text alignment in Graphics::drawStringAlgorithm()
Fix bug in Graphics::drawStringAlgorithm() painting invalid background color
Improve the generation of css::Query instances (see Sheet::query() member function)
Add styles to the Timeline widget

- Added support for styles to SkinTheme and skin.xml.
- Added app::skin::Style/Rule/BackgroundRule/IconRule/TextRule classes.
- Added app::skin::SkinPart class to refer to one part (BITMAP) of
  the skin sheet.
- Renamed app::skin::SkinPart to SkinParts as now SkinPart is a class.
Add padding to text style
Timeline: Add multiple frames selection
Merge branch 'timeline' into dev