[Feature] Add button to re-enter fullscreen

[CCuskley]

There have been some issues especially running on smaller tablets with the tasks exiting fullscreen mode. This may be difficult to totally prevent, but we would like to add a button e.g. to the top left that shows only when a task is out of fullscreen mode so that the user can easily re-enter fullscreen from withiin the task

Note we want this to be unobtrusive enough that we retain the ability to exit fullscreen during testing for our own purposes.