Make 'Merge' hotkeys consistent

I'm not sure if 'hotkey' is actually the technically correct term, but anyways:

When you press Alt + M to merge various vertices, you can press a further key to select the type of Merge. The problem is that these keys correspond to different merge types depending on whether or not your selection has an active element.

This is just generally inconvenient for using this tool, because you're generally not thinking about different keyboard shortcuts depending on how you selected elements of your mesh.

The same commands should require the same key-presses.