command-bar: Add ctrl-a/ctrl-e shortcuts

When adding ctrl-w/u/h to the command bar, Christian commented that
ctrl-a/e are also fairly widely used in command lines, for instance in
those tools using readline. Great minds think alike, I did wonder about
those shortcuts as well.

Implementing the whole list of readline shortcuts, _that_ would be
overkill !:)

https://bugzilla.gnome.org/show_bug.cgi?id=756594