Extend the window size after the tabbedSelectorSectionEnabled flag
is already set to true. Otherwise we extend the window and switch
to a normal from single column layout before showing the sidebar.
A lot of people didn't know how to bring the dialogs column back
after it was gone in a wide single column MainWindow.
Now we switch to a two column layout if the window allows it,
preserving the emoji / stickers / GIFs section in case it is shown.
John Preston