clipproperties: Work around bugs in the storemodel data setting code

We now simply store the selected effect directly on the storemodel and
use it so we avoid the use of pickle and other 'complicated' workarounds

Work around https://bugzilla.gnome.org/show_bug.cgi?id=737587

Conflicts:
	pitivi/clipproperties.py
ruler: Highlight only important timestamp elements
timeline: Fix dragging of second clip from the media library to the timeline

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=737634
timeline: Scroll to playhead when the zoom changes

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=734064