[Feature] Going fullscreen should hide the interface and fit images if not yet panned/zoomed

[larsmaxfield]

From #68 (comment).

Going fullscreen should:

• Hide the interface (but not filenames*)
• Fit images (if they have not yet been panned or zoomed**)

*Filename visibility is to become independent of interface visibility when #68 is implemented.

**Why? Because it is more intuitive that a user who has directly gone fullscreen after having added images would like to have those images fit fully in their screen.

[larsmaxfield]

ba49857 addresses fitting images