widgets: improve reuse of keyboard bindings, etc

Move more functionality into widgets.standard so that the rest of the
app picks up things like the keyboard navigation features.  This
implicitly adds the hotkeys to the rebase editor and various other
dialogs.

Signed-off-by: David Aguilar <davvid@gmail.com>
git xbase: add keyboard shortcuts for external diff

Use the standard shortcut for launching an external difftool.  Add the
action to the context menu.

Signed-off-by: David Aguilar <davvid@gmail.com>