Add toggle-fullscreen control with temp icons.

[fabiocaccamo]

Fix #189

[justvanrossum]

For icons, I think I prefer the "minimize" and "maximize" icons from https://tabler-icons.io/

We can use these as-is (filename and all), and put them in the /tabler-icons folder. Perhaps we need to tweak the stroke thickness and size to match the other tool icons (with CSS).

[justvanrossum]

Can you try adding CSS to tweak the icon stroke width for these? They are a bit heavy compared to the one directly to the left.

[fabiocaccamo]

I think that the icon that has a different stroke width is the "bullseye", actually the stroke width of the fullscreen icons is the same of plus and minus icons.

Changing the order of the icons should not have any impact on the visual harmony.

[justvanrossum]

It's still too heavy, even if I compare to plus and minus.