Add minefield scrollbars

[LewisGaul]

The main window shouldn't be made bigger than the available screen size - instead the minefield should use scrollbars.

[LewisGaul]

This is half completed - scrollbars are introduced when the window size is reduced (which is now made possible). However, the main window still needs an absolute maximum and to use scrollbars in the case the max is hit. Also, if there are scrollbars and the size of the board is reduced to be bigger than the current size, the scrollbars should remain rather than making the window bigger. Could also persist the state...

[LewisGaul]

Now tracked by #49