Remove gnome-nibbles-logo.svg

This logo is now unused
Validate appdata and desktop file at make check
Remove .desktop.in.in and BugBuddy info
Rely on intltool >=0.50 for gsettings translations
appdata: mark for translation
appdata: move the screenshot to a safer location
appdata: add a copyright line
appdata: reword the description a bit

* Nit: the word "manuever" twice in the same paragraph is weak

* Probably Nibbles will be restricted to two players per keyboard