Add "pin" button for Cars and Tanks

[sarge945]

Space Platforms have a "pin" button in their title bar, next to the close button.

This button should be added to Cars and Tanks as well, as they are commonly pinned, and the Alt-Rightclick shortcut isn't really documented anywhere outside of the keyboard controls.

This is just a minor QoL change.

[k1ng440]

Alt + right click on the car/tank/spider

[Stringweasel]

+1