screencastService: Allow for file extensions depending on encoder used

screencastService: Allow for file extensions depending on encoder used

Depending on the encoder we want to use a different container format and
therefore a different file extension. Right now this file extension is
forced to be webm, so shuffle things around a bit to make that more
dynamic.

Note that this also introduces removing for the old file created by the
recorder, otherwise it would create an empty "mp4" file every time it
falls back from "mp4" to "webm".

To be nice to external (ie. not gnome-shell) consumers of the
screencastService, let's not break the API completely, and detect the
".webm" suffix a little longer to not end up with weird file.webm.webm
filenames.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3211>