Hide maximization button in header bar if only a single tab is shown
By checking if the topmost ViewSplitter only contains a single tab, this change hides the maximization button for the header bar in case it is required to always be shown. The rationale behind this is that the maximization button would anyway be meaningless, as there is already only one single, maximized tab in display. The button should however be shown, if multiple tabs exist for that topmost ViewSplitter.