Transform: default to user-defined orientation

Previously we tried this but reverted (see 64d40c82c3)
because there wasn't a predictable set of keys to use global-space.

Now the keys are swapped:

- 'GX' always transforms in the user defined orientation.
- 'GXX' always transforms in global space.

As before 'GXXX' cycles back to disabling constraints.

This does have a down side that GXX won't be used for local-space
when the user has global space set.
Also, when global is the user-orientation, pressing GX and GXX
does the same thing.

Note: examples here use GX but could be any transform-mode/axis.

source/blender/editors/transform/transform_constraints.c

@@ -709,7 +709,7 @@ void setUserConstraint(TransInfo *t, short orientation, int mode, const char fte
 		{
 			char orientation_str[128];
 			BLI_snprintf(orientation_str, sizeof(orientation_str), "%s \"%s\"",
-			             IFACE_("custom orientation"), t->custom_orientation->name);
+			             IFACE_("custom orientation"), t->orientation.custom->name);
 			BLI_snprintf(text, sizeof(text), ftext, orientation_str);
 			setConstraint(t, t->spacemtx, mode, text);
 			break;