Issue #1184 - Merge Layers by overloading the layers dialog anchor button

The first of these commits merely adds a Merge Down button to take the place of the Anchor button whenever it is not relevant, and makes their menu items behave similarly, so that only one of them are visible at a time in the layers menu.

The second commit gives the Merge Down button some additional functionality, with modifier keys to merge layer groups (shift), as well as to merge all visible layers either from a dialog (ctrl), or just using the dialog's last values (ctrl + shift).

I wonder about at some point replacing the shift modifier with just switching the button over to merge groups when one is selected, though that would probably need a whole new accompanying icon, so I'm leaving that out of here for now.