With the addition of the 'disabled-extensions' key in commit ce1bee727,
the way extensions are enabled/disabled changed: Now a UUID is always
added to one list and removed from another.
Prepare for that by generalizing the relevant bits of the existing
enable/disable commands as helper functions.
https://gitlab.gnome.org/GNOME/gnome-shell/issues/1234
This replicates the most basic functionality of the existing
gnome-shell-extension-tool, albeit using a git/gio/gsettings
style command interface rather than plain options; this will
allow us to implement more complex commands that have options
on their own in the future.
https://gitlab.gnome.org/GNOME/gnome-shell/issues/1234