when okular active is not installed, it should not appear in the open with menu, so add TryExec to see if the executable exists
this is used instead of conditional install to make easier for distributions to package the desktop and active versions separately
BUG:314981
REVIEW:109047
splits the desktop file of the active app per mimetype,
as is done for the desktop version and for the part,
so only the ones that were actually built will be supported
Marco Martin
Script Kiddy