Make disabling the window before creating it actually work

Disabling a window before actually creating it ought to work, similarly
to hiding a window before creating it which can be used to avoid showing
the window on screen at all, even briefly. However it didn't under MSW
where the window was disabled from wxWidgets point of view, but not at
the MSW level.

Fix this by accounting for the enabled state in MSWGetStyle().

Closes #16385.

docs/changes.txt

@@ -102,6 +102,14 @@ Changes in behaviour which may result in build errors
   removing its name.
 
 
+3.1.?: (released 2012-??-??)
+----------------------------
+
+All (GUI):
+
+- Make disabling the window before creating it actually work.
+
+
 3.1.2: (released 2018-12-10)
 ----------------------------