Allow calling EnableSystemTheme(false) before creating the window

This is important as enabling the system theme results in changes to the
native ListView control appearance that are not undone later even if the
system theme is disabled. Notably, the item rectangle width is reduced
by 2 pixels when system theme is enabled and it's not increased to its
original value even when it's disabled later, resulting in gaps between
items through which the control background shows, for example. This also
makes items drawn using our own HandleItemPaint() slightly, but
noticeably, larger than the items using standard appearance, which looks
bad.

All these problems can be avoided if we skip enabling the system theme
in the first place if EnableSystemTheme(false) had been called before
creating the control, so support doing it like this and document that
this is the preferred way of disabling the system theme use.

Closes #17404, #18296.

interface/wx/systhemectrl.h

@@ -45,6 +45,27 @@
         };
    @endcode
 
+   Please also note that if you want to disable the system theme use in the
+   control that use it by default, it's best to do it before actually creating
+   the control as enabling the system theme can't always be completely undone
+   later. I.e. instead of
+   @code
+        // THIS CODE IS WRONG, DO NOT DO IT LIKE THIS
+        wxTreeCtrl* tree = new wxTreeCtrl(parent, wxID_ANY);
+        tree->EnableSystemTheme(false);
+   @endcode
+   prefer the following version:
+   @code
+        // Use default ctor to create the object, avoiding creating the window.
+        wxTreeCtrl* tree = new wxTreeCtrl();
+
+        // Then disable the system theme used by default.
+        tree->EnableSystemTheme(false);
+
+        // And only then actually create the window.
+        tree->Create(parent, wxID_ANY);
+   @endcode
+
    On non-MSW platforms this class currently does nothing but is still
    available, so that it can be used in portable code without any conditional
    compilation directives.