This commit adds initial packaging to build and distribute Purr Data
using Flatpak.

Add some comments and a few more useful desktop action examples to the main desktop file, so that the user understands how to adjust these if needed.

Replace pd-gui -> nw in the ForceQuit actions, which is the proper name of the GUI program on Linux.

These aren't going to be substituted in desktop actions anyway, because
desktop actions are supposed to be chosen from a menu, not being dragged to.

We do this to prevent extra arguments to be passed from the DE.

At least on KDE, it seems that the desktop file itself gets passed if the
action is invoked by right-clicking the icon on the desktop (this doesn't
happen with panel icons). Maybe this is a bug in KDE, but we need to prevent
this, because otherwise Purr Data tries to open the desktop file as a patch,
which of course will cause a bunch of ugly error messages in the main window.

Fix the Debian control files once again, since the dependency auto-detection needs a Depends line in there.

Make INCREMENTAL work on Windows (must be passed as make variable, as shell variable is not inherited in submakes).