Now the history of recently used colors is not updated when selecting
in the color dialog anymore, but when a color is actually used.
Tools supported right now are: Ink, MyPaint brush, all PaintBrush tools,
bucket fill and eraser (background color on non-alpha drawables).
Moreover from now on, colors already saved are properly moved to first
position when reused.
Add a generated palette which contains the color history. For now it's
only updated when the color dialog's color history gets updated, but
should be updated whenever a color is chosen in any way.
