extensionSystem: Sort extensions before loading

extensionSystem: Sort extensions before loading

When an extension is disabled, we "rebase" all extensions that
were enabled afterwards, in case they override the same method.

This can be fairly expensive, in particular when disabling many
extensions at once, for instance when locking the screen.

Help with that by sorting extensions by the number of session-modes,
as extensions that support multiple modes are less likely to be
disabled on session mode changes.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2851>