multimedia/{py-,}mlt{-qt5,}: rename to multimedia/{py-,}mlt6{-qt5,}:

• To make it possible to easily import mlt-7 move the current port to a versioned directory.

• Add Mk/Uses/mlt.mk to make it easily possible to depend on a given mlt version:
  
  USES=mlt:6
  
  will get you a dependency on multimedia/mlt6, while

USES=mlt:7

will get you a dependency on the soon to be added multimedia/mlt7

• Switch the port to use cmake as a build system
• While here, provide some more options to make it possible to have a leaner package if required

• Bump consumers after rename of the dependency