Refactor text to/from double conversion in wxSpinCtrlGenericBase.

Refactor text to/from double conversion in wxSpinCtrlGenericBase.

The code always used ToDouble() and Format("%g") which was a bit strange for
integer-valued wxSpinCtrl. Move the conversions to their own virtual functions
for clarity, perhaps correctness and, especially, flexibility as they will be
overridden in wxSpinCtrl soon.

Also move wxSpinCtrlGenericBase::m_format to wxSpinCtrlDouble as the base
class really doesn't need it at all.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72412 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775