Fix issue double-clicking on pre-selected pictures
Move generation of .desktop file to data/Makefile.am and use that Makefile

Because of a mistake, this Makefile was not being generated by
Autoconf, nor being used at all before, causing that the AppData
file was not being installed at all.
Don't hardcode the path to pixmaps in frogr.desktop.in.in
Include the appdata file when creating the rpm package