Handle pov property to filter the POV selector in the propertiesView and the characters list in the storylineView

Hi,

The purpose is to handle the POV property set on characters to filter the character list in the storyline view and in the POV selector of the properties view.

I'm not familiar with Python or QT, so these may not be the most elegant and/or efficient solutions, but based on what I've tested, it seems to work.