Allow entering minus sign in wxMSW wxSpinCtrl if needed.

Allow entering minus sign in wxMSW wxSpinCtrl if needed.

Only restrict input to wxMSW wxSpinCtrl to digits only if it doesn't support
negative values, otherwise the user can't enter any negative numbers. As there
is no built in support for entering digits and minus sign only, simply allow
all characters to be used if the negative numbers must indeed be possible to
enter.

Closes #11082.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61754 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775