[MainWindow] Set columns clickable only if they are sortable
[MainWindow] Set columns clickable only if they are sortable

Fixes #24 (although whether "unclickable" means "greyed out" depends
on the theme).

Signed-off-by: Jussi Kukkonen <589e25658b1bfd3646cd0c60eea9e9594b5e0b5b@intel.com>

lferrandis/dleyna-control,rmerlino/dleyna-control,01org/dleyna-control,markdryan/dleyna-control,jku/dleyna-control