GApplication: add env variable to override app ID
@3v1n0
Submitted by Marco Trevisan Link to original bug (#788207)
Description
Sandboxed apps in snaps or flatpaks might want to have different app IDs to allow the installation of multiple versions of the same app, thus having an unique APP-id which is different from the one that the publisher did.
So introducing: G_APPLICATION_FORCE_OVERRIDE_APP_ID
Which allows to force the app id overriding, no matter if it has been allowed via flags or not.
Resources path should not be changed unless it's not previously set.
Command line argument has precedence anyway in case is set and allowed.