ReaScript updates

+Updated description for some API
+Renamed stretch_marker to item_stretch_marker in BR_GetMouseCursorContext()
+Small visual cleanups to other parts of the code
Zooming fixes
+All zoom actions now obey track sizes set by custom themes (things like small/medium/full track height)
+Actions that zoom vertically to selected tracks/items never show track envelopes of the last selected item/track
+Fixed save/restore arrange view actions not working on envelopes with high point count (Closes #660)

+Deprecated SWS_TrackEnvelope(s), BR_Envelope is much more complete and doesn't have problems with big envelope chunks
+Completely removed TrackHeightFromVZoomIndex() from Zoom.cpp since it wasn't obeying custom themes and better alternatives are available in BR_Util.h
+Comments wankage in BR_MouseUtil.h
Merge branch 'next' of https://github.com/Jeff0S/sws into next
Localization fixes.