Adjust entry width of wxSpinCtrl in wxGTK to its range

Ensure that the entry is always (just) big enough to show any value
valid in this spin control.

This also ensures that GetBestSize() doesn't need to be overridden to
use GetSizeFromTextSize() any longer as the best size will be determined
correctly by GTK itself.

include/wx/gtk/spinctrl.h

@@ -71,7 +71,9 @@ protected:
     void GtkDisableEvents() const;
     void GtkEnableEvents() const;
 
-    virtual wxSize DoGetBestSize() const wxOVERRIDE;
+    // Update the number of digits used to match our range (and base).
+    void GtkSetEntryWidth();
+
     virtual wxSize DoGetSizeFromTextSize(int xlen, int ylen = -1) const wxOVERRIDE;
     virtual GdkWindow *GTKGetWindow(wxArrayGdkWindows& windows) const wxOVERRIDE;