Never enable wxSpinCtrlGenericBase window itself at underlying toolkit level.

We create wxSpinCtrlGenericBase window disabled at underlying toolkit level
(but enabled at the level of wxWidgets API, of course), but calling Disable()
and Enable() on it re-enabled it resulting in loss of mouse events under
Windows.

Don't re-enable it (again, at the low-level toolkit level only) in its
Enable() any more but keep this window itself always disabled.

Closes #12045.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64302 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775