Added Radio buttons support for the Marble Legend Browser

GCI task #5890891378065408
REVIEW: 114031
Minor code (style) improvements
Fix EBN warnings
Last sync is shown in preferences

Time of the last sync is shown now inside the Sync preferences pane
as status label

REVIEW: 114041
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.

BUG: 327405
REVIEW: 114054
show decoration widget
FindQtMobility with components
QSystemInfo