Fix for Editor Fullscreen on Sequencer

The sequencer 'display' area is a region on its own, so we can't hide
the preview regions. The only problem is that the <-> resize icon shows
in the main region, so you only see it over the tracks region in the
sequencer, I'm not addressing this though.

source/blender/editors/screen/screen_edit.c

@@ -1906,7 +1906,6 @@ ScrArea *ED_screen_state_toggle(bContext *C, wmWindow *win, ScrArea *sa, const s
 
 				if (ELEM(ar->regiontype,
 				         RGN_TYPE_UI,
-				         RGN_TYPE_PREVIEW,
 				         RGN_TYPE_HEADER,
 				         RGN_TYPE_TOOLS))
 				{