Add new FullScreenMngr class dedicated manage full-screen action to digiKam...

Add new FullScreenMngr class dedicated manage full-screen action to digiKam KXMLGuiWindow instances.
This class factorize a lots of code when window is turned on/off to full-screen mode.
This fix problem about visibility restoring with status-bar, menu-bar, and thumb-bar.
First managed digiKam main window is Image Editor.
TODO :
- manage full-screen mode of main windows AlbumGUI, BQM, ImportUI, and LT, through FullScreenMngr.
- manage visibility of side-bars.
- provide a common settings widget to control visibility management in Configuration dialog.
- patch Configuration dialog to provide a fine control of visibility management in full-screen mode for each digiKam main window.

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