We were previously relying on optionstate.filenames being non-NULL to
avoid starting thumbnailing when the user opened the player with a
video, but that actually caused doubly-adding those files passed on the
command-line, and didn't work when the Open() GApplication method was
used.

Use a separate option to keep track of whether to start populating the
grilo icon view and thumbnailing, so that thumbnailing doesn't hamper
playback performance.

https://bugzilla.gnome.org/show_bug.cgi?id=746165