Blender Git Loki

VSE: Add option to select handles with box selection

Patch adds an "Handle" option to the `SEQUENCER_OT_box_select` operator,
that allows to select the handles instead of whole strips.
Feature is mapped to Alt key modifier

A difference from the proposed design in T70730 is that covering the entire strip with the box actually selects both handles.

Reviewed By: iss

Differential Revision: https://developer.blender.org/D6372