Add revert-buffer support to the mu4e main buffer

The mu4e-main buffer now supports the traditionnal revert-buffer interface: hit g to revert the buffer.

I implemented this so that I could toggle queuing from any mu4e buffer, in particular while composing an e-mail, without switching to the main mu4e buffer.  To this end one has to add a keybinding to the desired keymaps. A commented-out exemple can be found at the end of the mu4e-main.el file.

Only very minor changes were needed, and I only touched the mu4e-main.el file. Most changes are tagged with a ";; NEW"  comment.