Introduce a new standard edit action group "edt" for editing actions
such as copy and undo, separate from the "win" window action group, so
that editing actions can be scoped to specific widgets and overridden
by children. Add new Action namespace with sub-namespaces for the
app, win and edt namespaces and move consts from GearyApplication there.
Update call sites and UI files, use consistent approach to setting up
action groups across all classes that use them.
Show GNOME settings when adding a supported GOA service type and
it is installed, otherwise show Geary's custom password-based impl.
For GOA accounts, hide the Remove button, and enable opening the
account in GNOME Settings from the server pane.
This allows the panes themselves to manage their own headers, command
stacks, and so on. Break out the account list pane out so it's handled in
the same way as the others.
