Implement wxAuiNotebook::GetBestSize()

Compute the best size of the notebook, taking into account all the
different layout possibilities, and add a test checking that this works
as expected.

Closes https://github.com/wxWidgets/wxWidgets/pull/1085

docs/changes.txt

@@ -116,6 +116,7 @@ All (GUI):
 
 - Fix wxInfoBar close button size in high DPI (Stefan Ziegler).
 - Make disabling the window before creating it actually work.
+- Implement wxAuiNotebook::GetBestSize() (Sebastian Walderich).
 - Allow changing tooltip text for button allowing to enter a new string
   in wxPGArrayEditorDialog.