Add Palette::findExactMatch() member function
Fix warning message on debug
Fix for PaletteView: redraw it when the palette is changed
Change current palette after undo/redo
NewFile command: Don't change palette in grayscale images
ColorSelector: Add WarningIcon to add colors to the palette

Simplified the color selector UI. Now RGB/HSV colors are shown correctly
and a new warning icon/button is visible when the color is not part of
the current palette. This button can be used to quickly add the new entry
to the palette.