#47 Add toolbar button to enter Fullscreen mode

[FeRD]

This world's-simplest (literally one line) patch adds a toolbar button to initiate Fullscreen mode (partially addressing feature request ticket [#86]). I placed it to the left of the Slideshow icon (after the separator, so that it's paired with Slideshow), which felt like the most natural place.

[Ark]

Thank you very much for the patch. I just pushed a commit that includes it.