Use generic spin control itself as parent for its children.

Use generic spin control itself as parent for its children.

This fixes a problem with using wxSpinCtrlGeneric in toolbars under wxOSX,
using the toolbar itself (i.e. the parent of the spin control) as parent for
the children didn't work there and no windows were visible at all.

Also use wxNavigationEnabled as base class of wxSpinCtrlGeneric to fix
keyboard navigation.

And override SetBackgroundColour() to set it for the text control part of the
spin control only.

Closes #15016.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73630 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775