Implement GetSizeFromTextSize() for wxSpinCtrl.

Implement it for the native MSW and GTK versions and the generic one used in
the other ports and also for wxSpinCtrlDouble under MSW.

Also test this function in the spin page of the widgets sample.

Closes #14840.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72995 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/generic/spinctlg.h

@@ -108,6 +108,7 @@ public:
 protected:
     // override the base class virtuals involved into geometry calculations
     virtual wxSize DoGetBestSize() const;
+    virtual wxSize DoGetSizeFromTextSize(int xlen, int ylen = -1) const;
     virtual void DoMoveWindow(int x, int y, int width, int height);
 
 #ifdef __WXMSW__