Added support for calculating the "desired" size of UIElements (how much space they want to use).
Moved the scrollbar into its own widget, out of the ScrollableArea layout.
Implemented scrolling of scrollable areas with the mouse wheel.
Firmed up some UIScrollbar behaviour with clamping of value to the range.
Removed some duplicate code to simplify skin system maintenance.
Cleaning up UIStyle and UISkin
Merge branch 'develop' into nui
Tidy up of canvas-skin interaction.