SwatchesPanel cleanup
A major simplification of the SwatchesPanel code. Everything should be functionally identical as before, with two exceptions:
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I removed the "showclose" option since every panel already has a close "X" button at the top-right.
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A proper GtkMenuButton replaces the EventBox-based menu popper in the swatches panel. This change apparently also fixed for me the issue with the overlapping scrollbar (probably because of the increased size of the button)