Cleanup: Get rid of ScrArea.butspacetype syncing with ScrArea.spacetype

The only real reason we need `butspacetype` is while switching areas, where we
need to delay the actual switch to the RNA _update callback since only there we
can access context.
So instead of trying to sync it with `spacetype`, only set while needed and
unset it afterwards (as in set to `SPACE_EMPTY`).

This should also allow us to re-use `butspacetype` in versioning code when
trying to read removed editors. It'll store the space type value of the removed
editor which we can then use on versioning.

For backwards compatibility, we store `butspacetype` with the value of
`spacetype`.

source/blender/makesrna/intern/rna_screen.c

@@ -159,8 +159,9 @@ static const EnumPropertyItem *rna_Area_type_itemf(bContext *UNUSED(C), PointerR
 static int rna_Area_type_get(PointerRNA *ptr)
 {
 	ScrArea *sa = (ScrArea *)ptr->data;
-	/* read from this instead of 'spacetype' for correct reporting: T41435 */
-	return sa->butspacetype;
+	/* Usually 'spacetype' is used. It lags behind a bit while switching area
+	 * type though, then we use 'butspacetype' instead (T41435). */
+	return (sa->butspacetype == SPACE_EMPTY) ? sa->spacetype : sa->butspacetype;
 }
 
 static void rna_Area_type_set(PointerRNA *ptr, int value)
@@ -197,6 +198,9 @@ static void rna_Area_type_update(bContext *C, PointerRNA *ptr)
 			ED_area_newspace(C, sa, sa->butspacetype, true);
 			ED_area_tag_redraw(sa);
 
+			/* Unset so that rna_Area_type_get uses spacetype instead. */
+			sa->butspacetype = SPACE_EMPTY;
+
 			/* It is possible that new layers becomes visible. */
 			if (sa->spacetype == SPACE_VIEW3D) {
 				DEG_on_visible_update(CTX_data_main(C), false);