Hide the menu bar by default
This commit hides the menu bar for new installations by default. This is done by introducing a setting to save if Gwenview was ever started before. Unfortunately this check doesn't work for upgrading users so the menu bar is hidden only if a hamburger menu action is currently in the toolbar which can only be the case if it is a new installation. Motivation: With the hamburger menu now visible on the toolbar by default, we can now hide the menu bar by default without having to fear that users won't be able to access Gwenview's features. Hiding the menu bar by default makes sense because Gwenview is supposed to be a simple image viewer meaning that users shouldn't need fast access to the plethora of actions in the menu bar. Hiding the menu bar by default increases the default area available for showing an image and directs users to be instead more aware of the select view actions which remain visible on the UI.