Graph builder: Make duplicated filter detection stricter.

This let us blacklist filters more easily.
Blacklist some filters that are known to be incompatible with DVB.

This includes the internal video decoder for now (refs #730).
Cosmetics: add missing brackets and consistently use .0f instead of .f.
Some testing on SetAlwaysOnTop. (not working)
Apply the "Repeat" option after "Play next in the folder". (WIP)

The repeat option will now take effect after the next file in the folder was played (similarly to playlist items).

This commit fixes #978.