Various improvements to desktop file

Version number needs to be the version number of the desktop file format. The exec command needs to accept multiple files (capital F). The tryexec needs to accept no files. Added a comment.

Contributes to issue CURA-730.

cura.desktop

@@ -1,12 +1,12 @@
 [Desktop Entry]
-Version=15.06.01
+Version=1
 Name=Cura
 Name[de]=Cura
 GenericName=3D Printing Software
 GenericName[de]=3D-Druck-Software
-Comment=
-Exec=/usr/bin/cura_app.py %f
-TryExec=/usr/bin/cura_app.py %f
+Comment=Cura converts 3D models into paths for a 3D printer. It prepares your print for maximum accuracy, minimum printing time and good reliability with many extra features that make your print come out great.
+Exec=/usr/bin/cura_app.py %F
+TryExec=/usr/bin/cura_app.py
 Icon=/usr/share/cura/resources/images/cura-icon.png
 Terminal=false
 Type=Application