Add a new `changed` signal to UndoableStore that gets fired whenever
the data changed, but only once after an undo or redo block.

This provides both a simpler API to watch the model for changes, and
also avoids getting too many change notifications on undo/redo.

Simply don't set the value if it's the same, so it avoids creating an
undo entry and also avoids refreshing the theme.

Use a custom cell renderer for rendering the color column, so  that
the color still remains visible when the row is selected.