Toggle fullscreen button in the header bar

[vanillajonathan]

GNOME's image viewer Loupe have a toggle fullscreen button in the header bar. Maybe SphereView should too?

[dynobo]

This is an easy one: the action already exists and is currently triggered by F11. One only need to add the button and connect it to the same action.

Edit: one thing I'm unsure how to solve: When having fullscreen button in headerbar, we should remove it from the bottom bar to avoid redudancy. However, in fullscreen, the headerbar is not visible -> No button to exit fullscreen.
Showing the fullscreen button in the bottom bar only during fullscreen also seems bad UX... Not sure how this could be solved without adding too much complexity (like duplicating the headerbar or such)...