Changes:
- Added ui::ScrollableViewDelegate so ui::ScrollBar can be child of
a non-ui::View widget.
- Added a generic ui::setup_scrollbars() utility to locate
horizontal/vertical scrollbars depending on the scrollable area and
the available viewport area.
- Replaced Timeline::m_scroll_x/y ints with m_hbar/m_vbar widgets.
- Added transparent scrollbar look & feel.
- Added a "hover" state to mini-scrollbars used in sprite editors.
