install a .desktop file for each Marble variant (Qt, Mobile, KDE)

It's technically possible to install all .desktop files in parallel.
However, since all files have the same user-visible content, multiple seemingly identical entries will appear in the XDG menu which will start different variants of Marble.
Hence, we need to rely on packagers to select the appropriate .desktop files for the packaged Marble binaries.
(cherry picked from commit 0a88e37a)

BUG: 327405
REVIEW: 114054
FIXED-IN: 1.7.0 (KDE 4.12.0)