command-bar: Make sure any old text is cleared when showing

https://bugzilla.gnome.org/show_bug.cgi?id=740629
command: Add CommandProvider::complete signal and gb_command_manager_complete

This adds the initial scaffolding to get commandline completion in
the command bar. It just lets you fill in possible completions given
some existing text.

https://bugzilla.gnome.org/show_bug.cgi?id=740629
command-gaction-provide: Implement action name completion

This completes on the available registered actions.
For now only names are completed on, no parameters.

https://bugzilla.gnome.org/show_bug.cgi?id=740629
command-bar: Implement tab completion

When you press tab we expand to the longest common prefix from, and
if no common prefix exists we show the available matches in a list
(multiple tabs scroll in the list).

https://bugzilla.gnome.org/show_bug.cgi?id=740629