Politically correct terrible consequencer changes

This patch includes the work done in the terrible consequencer branch
that hasn't been merged to master minus a few controversial and WIP
stuff, like strip parenting, new sequence data structs and cuddly
widgets.

What is included:

* Strip extensions only when slipping. It can very easily be made an
option but with a few strips with overlapping durations it makes view
too crowded and difficult to make out.
* Threaded waveform loading + code that restores waveforms on undo (not
used though, since sound_load recreates everything. There's a patch for
review D876)
* Toggle to enable backdrop in the strip sequence editor
* Toggle to easily turn on/off waveform display
* Snapping during transform on sequence boundaries. Snapping to start or
end of selection depends on position of mouse when invoking the operator
* Snapping of timeline indicator in sequencer to strip boundaries. To
use just press and hold ctrl while dragging.

Reviewers: campbellbarton

Differential Revision: https://developer.blender.org/D904

source/blender/windowmanager/WM_api.h

@@ -398,6 +398,7 @@ enum {
 	WM_JOB_TYPE_CLIP_SOLVE_CAMERA,
 	WM_JOB_TYPE_CLIP_PREFETCH,
 	WM_JOB_TYPE_SEQ_BUILD_PROXY,
+	WM_JOB_TYPE_SEQ_BUILD_PREVIEW,
 	/* add as needed, screencast, seq proxy build
 	 * if having hard coded values is a problem */
 };