CMake: Rename desktop file to the conventions made by Freedesktop

While working on the flatpaks, I got complaints from flatpak-builder that org.mavlink.qgroundcontrol.desktop was not found.
Since flatpak is made by the Freedesktop people, I strongly believe that this is actually an convention, which they want to enforce with their flatpak ecosystem.
Nevertheless, just doing so, so the builder stops complaining at least.

Signed-off-by: Thomas Karl Pietrowski <thopiekar@gmail.com>

CMakeLists.txt

@@ -198,9 +198,8 @@ install(
     DESTINATION ${CMAKE_INSTALL_DATADIR}/qgroundcontrol
 )
 install(
-    FILES ${CMAKE_SOURCE_DIR}/deploy/qgroundcontrol-clean.desktop
+    FILES ${CMAKE_SOURCE_DIR}/deploy/org.mavlink.qgroundcontrol.desktop
     DESTINATION ${CMAKE_INSTALL_DATADIR}/applications
-    RENAME qgroundcontrol.desktop
 )
 install(
     FILES ${CMAKE_SOURCE_DIR}/resources/icons/qgroundcontrol.png