Blender Git Loki

Markers: Click-drag in empty area to box select markers

To make this work, other changes were necessary:
* To select a specific marker you have to click more exactly on the icon.
* Moving markers with click-drag only works when starting on a marker.

Additionally this patch implements that all markers are deselected,
when the user clicks in an empty area.

Reviewers: billreynish, brecht

Differential Revision: https://developer.blender.org/D4975