Make the margin between wxSpinCtrlGeneric sub-windows compatible with MSW.

Make the margin between wxSpinCtrlGeneric sub-windows compatible with MSW.

Native MSW wxSpinCtrl uses a single pixel margin between the text part and the
spin button while the generic version used 2 pixels which resulted in a
slightly different appearance (see #12767).

Use the same margin as MSW now in the generic version too, it should be fine
for the other platforms as well and if it isn't we can always tweak them
later.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66444 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775