tabwidget: Properly clean up tabs when hidden

From on_tab_removal:
    # If one tab is removed, the whole tabwidget will die..
    # This is easy usually the desired behavior.

We did not, however, call hide() for the currently visible tab and just
killed the tabwidget around it. This meant that for instance scheduled
events referring to the old tab (think refresh calls) were still valid
and crashing later.
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