kateviewmanager: Show welcome page as for new windows when all tabs closed

• When the option to show the welcome page for new windows is set, if all tabs are closed this will now show the welcome page rather than a new document.
• If a welcome page is already open, it is now activated rather than doing nothing or opening another - also for the welcome page action.

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The option to quit when all tabs are closed (default in KWrite) is preserved.

I believe this is a good change, as when all tabs are closed, there is the expectation for the application to behave as if it had just started, showing the welcome page if enabled.