Fix snap package (Crash, printing)

Another step in the maybe never-ending story to get the "snap" package format working. #5414

Relevant only to the master branch:

• Remove libraries that lead to crash due to conflicts with the libraries provided by the gnome snap extension. (In my opinion, the snap packaging/build system has a serious design flaw here: The gnome snap extension is not compatible with the Ubuntu GTK4 packages. The gnome extension compiles all stuff on their own. We would in theory have to do the same for everything they don't supply, compiling it against whatever library versions they use. In the end you would end up doing everything yourself like in the stone age before package management was invented :-D .)
• Correct syntax (quoting etc.) of launcher script, fix GS_LIB variable

Also relevant for backporting to 1.4:

• Fix printing and print preview

I tested this locally.

Main issue: #5414

Fixes #1734 (closed)

For backport to 1.4.x see !6951 (merged)