Fix wxGenericCollapsiblePane best size calculation

DoGetBestSize() actually calculated the best client size and not the
full size, as it didn't take the pane border into account.

Fix this in the simplest possible way, by just renaming the function to
DoGetBestClientSize() instead.

This ensures that the pane is actually big enough to show its contents,
without cutting off the text shown in its header.

Closes #18515.

include/wx/generic/collpaneg.h

@@ -73,7 +73,7 @@ public:
 
 protected:
     // overridden methods
-    virtual wxSize DoGetBestSize() const wxOVERRIDE;
+    virtual wxSize DoGetBestClientSize() const wxOVERRIDE;
 
     int GetBorder() const;