Allow maximising the main window

[LewisGaul]

This is unintentionally possible when running through an x-server (XLaunch), but actually it can be nice to block everything else out.

This state should be persisted over restart.

Requested by Kyle

[LewisGaul]

This is kind of horrible to get working properly :)

• Maximise button should be enabled (easy)
• Default size should be the tight fit around the board (or with scrollbars if the board is bigger than the max window size)
• When changing the board size (via dimensions or button size):
  • If the window is maximised it should stay maximised
  • If the window is not maximised and does not have any scrollbars it should be set to the default size
  • If the window is not maximised and has scrollbars it should be set to:
    • the tight fit around the board for dimensions where this is smaller than the current window size
    • unchanged for dimensions where the size is bigger or equal to the current window size
• Ideally it would not be impossible to increase the window size beyond the tight fit. However, the naive approach prevents maximising the window [on Windows].
  • A less satisfactory alternative would be to reset to the default size when selecting the current difficulty (not including selecting a custom board of the same size), which can be done with a single keyboard button press (e.g. 'b').
• Persist window state:
  • Whether maximised
  • The dimensions of the window (or displayed size of the minefield?)
• The name bar should be strictly along the bottom, with padding being around the minefield frame such that the minefield is in the center of the space

[LewisGaul]

Enabled in beta options in PR #125, although buggy and state not persisted.