colors were a mixture of 4bpp integers (0=white, 15=black) and
fractional blackness levels (0=white, 1.0=black) before. This is
now unified to use the color specification of the Blitbuffer API.
- Refactor FileDialog and HightlightDialog into ButtonDialog
- Create a new ButtonDialog when a file in the History dialog is hold,
offering an option to delete the history entry.