Update the UI to match current guidelines

I did quite a lot of changes at once:

• Use the GMenuModel XML format for all menus (one .ui file per menu)
• Use GtkPopover for all menus
• Use GSimpleAction for menu actions and keyboard accelerators
• Fix the shift+f10 shortcut
• Put the appmenu in the window as new guidelines want
• I used a .ui file for the headerbar (there was a TODO about that in the code)

Potential drawbacks:

• i didn't know how to hide a popover menu item, so they're just disabled if they can't be used, so the partition/volume menu is big as hell.
• i didn't add a shortcut window, so users have no indications about that (but who is formatting his drives from the keyboard anyway ?) Fixed
• i wasn't sure what to do with "attach a disk image" and "new disk image" so my design doesn't respect the weird temporary guidelines they did for now Fixed, and i added new shortcuts about it
• i didn't test with an old version of udisks2 because i don't have one

It's the first time i do such a big contribution, and I'm quite not comfortable with C, so there are probably memory leaks, you really should review it before merging.