Blender Git Loki

Keymap: Drag in empty area to box select in animation editors

This is a patch for the default keymap in Blender.
It relates to the Dopesheet, Timeline, Graph Editor, NLA and Sequencer
Currently, in these editors, dragging outside of your selections does nothing.

This patch makes it so dragging outside the selection does a box select operation.
It is consistent with how the Node Editor works, as well as the 3D View, if you use the gizmo overlays.

Differential Revision: https://developer.blender.org/D4799

Reviewers: campbellbarton