Add second level of popups

Group commands under an annex dispatch popup. This has a few advantages:

- Allows commands to be expanded without the popup becoming too
  cluttered.
- Options that only apply to a limited number of commands (e.g.,
  `--content') aren't shown for all commands.
- After the switch to the new magit-popup.el, this will allow default
  options to be activated for certain groups. (The main one I have in
  mind is `--auto' for actions on all files.)

The tradeoff with this is that it adds one more key to the sequence. For
a few common commands, a convenience key can be added under the annex
dispatch (which is what is done now for `magit-annex-get-all').
Add commands for actions on all files
Rename git annex add commands
Move push and merge under annex popup

The main reason for having the push commands under the main magit
pushing menu was that they share options with the push commands because
they're just pure push commands. However, I think it's less surprising
to have everything under the main annex menu.

`git annex merge' was also under a non-annex menu, but it's not a pure
merge command and doesn't share options with `git merge`, so I'm not
sure why I put it under the merge menu.