Move KColorScheme instanciation out of the paint path.

REVIEW:108958
use TryExec to not appear when not installed

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
SVN_SILENT made messages (.desktop file)
SVN_SILENT made messages (.desktop file)