Make Pharo Launcher GTK-ready app runnable on Morphic

[demarey]

Since it is not yet possible to develop a GTK app and run it from the same image, it is important to allow to run a GTK-ready spec app with Mrophic backend so that early testing can be done in Morphic backend.

[estebanlm]

technically, is not possible in macOS (while it is fine in linux or windows :P), but I get it ;)