Toolbar and menu action relayout

Implements the changes proposed some time ago on the mailing list.
In the main toolbar, there are the four main windows: IE, Light table, BQM, Camera import;
then the three modes of the main panel: thumbnails, preview, map; then slideshow and full screen.
In the menu bar, the Image menu is resorted. There is a "Browse" menu which offers access
to the left side bar places, marking them as the primary access to the user's pictures.
Some more changes, internal cleanups.
The main window actions offer some automatic guessing, like the "import" button
automatically opening the last plugged camera (need to test), or the light table button
using some heuristics whether to clear the light table before.

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