Fix snap build

Quick-and-dirty changes to make the snap (Ubuntu-Linux snap packaging system) build again for 1.5-dev. It builds and I can successfully start and somewhat use Inkscape, but there are many open isues left:

• The close/minimize buttons are on the wrong side of the window title bar (left instead of right)
• Saving / Opening doesn't really work
• Extensions hang
• Patches specific for snap had to be disabled, probably this causes the above issues (IMHO these patches should be merged into the main code, probably with an if-condition depending on some environment variable)
• Dependencies for the extensions are probably incomplete
• The snap build configuration is quite a mess, many old things need to be checked and probably deleted
• Sometimes there is a warning about libproxy but I could not reliably reproduce this. (Only once after installing the snap?)

I would ask to merge this so that someone else can continue fixing the rest. I would then set up a separate issue for the remaining points.

Related: #5414