This improves the modifier header menu in a few ways:

1. Adds "move to top" and "move to bottom" buttons.
2. Adds a checkmark icon for "apply"
3. Makes it narrower, the text is closer to the dropdown icon.

This does not do a few other things:

1. Add shortcuts to the menu (see D8031)
2. Change the header layout (see D8037)
3. Add "move up one" and "move down one" buttons. I don't believe this is worth the space they take up for the vast majority of people. Dragging up and down one is quite easy, especially with the new region scrolling.

I had to fix a little issue with the max width for menus for the narrower menu to work.