Terminal: remove fullscreen tooltip

Whenever using Terminal in fullscreen mode, there is a permanently on
tooltip with the shortcut to exit fullscreen mode wherever you leave
your mouse. This is super annoying and as a result I find myself rarely
using fullscreen mode.

Remove the tooltip, and instead add a button to the right of the tab bar
to exit fullscreen mode, as done in WonderBrush and WebPositive.

Change-Id: If42f038da3d644970de6214a31195882d4fc7eee
Reviewed-on: https://review.haiku-os.org/c/1507
Reviewed-by: waddlesplash <waddlesplash@gmail.com>