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

release/scripts/startup/bl_ui/space_sequencer.py

@@ -71,6 +71,7 @@ class SEQUENCER_HT_header(Header):
         row.prop(scene, "lock_frame_selection_to_range", text="", toggle=True)
 
         layout.prop(st, "view_type", expand=True, text="")
+        layout.prop(st, "waveform_draw_type", text="")
 
         if st.view_type in {'PREVIEW', 'SEQUENCER_PREVIEW'}:
             layout.prop(st, "display_mode", expand=True, text="")
@@ -82,6 +83,7 @@ class SEQUENCER_HT_header(Header):
 
             layout.separator()
             layout.operator("sequencer.refresh_all")
+            layout.prop(st, "show_backdrop")
         else:
             if st.view_type == 'SEQUENCER_PREVIEW':
                 layout.separator()
@@ -716,6 +718,7 @@ class SEQUENCER_PT_sound(SequencerButtonsPanel, Panel):
     def draw(self, context):
         layout = self.layout
 
+        st = context.space_data
         strip = act_strip(context)
         sound = strip.sound
 
@@ -734,7 +737,9 @@ class SEQUENCER_PT_sound(SequencerButtonsPanel, Panel):
 
             row.prop(sound, "use_memory_cache")
 
-        layout.prop(strip, "show_waveform")
+        if st.waveform_draw_type == 'DEFAULT_WAVEFORMS':
+            layout.prop(strip, "show_waveform")
+
         layout.prop(strip, "volume")
         layout.prop(strip, "pitch")
         layout.prop(strip, "pan")