ColorListWidget: Remove unused colors

[dl3sdo]

Add a button to the Colors window that allows to remove colors which are not used by any symbol. Before the removal a popup window offers to show the list of colors to be removed and asks for a confirmation.

Closes #381

[dl3sdo]

Remove_unused_colors.mp4

[dg0yt]

I wonder if it was more useful to have an action to just select unused colors. This would allow to see them in context. And the Delete button could remove all selected colors.

[dl3sdo]

Currently the color window does not allow to select more than one color. A multi-color selection increases the complexity if the user does not just press the delete button but instead decides to move or add a color: you would need to either disable those buttons or handle it in a special way (move all selected, but add after first selected or refuse to add?). Can multi-color selection be limited to the select unused button action to avoid additional complexities?
And you replace one button (I admit that Cleanup is not the most intuitive label but the shortest one, Remove unused could be a good alternative) by another one.

I doubt that the additional effort is worth the slightly increased gain in giving an easy overview, especially for a feature which is in most cases never or only rarely used.
The implementation itself was easy after implementation of #2222.

[lpechacek]

Currently the color window does not allow to select more than one color.

As a side note, we've got enhancement request #2175 which is about moving multiple colors at once. I started the work but it's not yet finished. If nothing else, I've got vector versions of the green arrows for "move color up/down" and "move color to the top/bottom". I can share the individual bits if needed.