Implement split cell mode

[LewisGaul]

Fixes #41

This supercedes #147 (and #148, which was accidentally closed).

• [x] Fix tests
• [x] Get chording working
• [x] Detect game being won
• [x] Add menubar button to switch mode
• [X] Disable highscores
• [X] Support creating boards in split-cell mode
• [x] Implement save board (store game mode) and make load board switch mode
• [x] Set 50% mine defaults for standard game sizes
• [x] Prevent custom boards with odd dimensions
• [x] Fix issues with drag-select flagging (also see #128)
• [x] Calculate 3bv, remaining 3bv, ...
• [x] Make highscore settings take game mode into account when opening highscore window
• [x] Make sure difficulty radiobuttons are marked correctly
• [x] Look into performance (e.g. recalculating numbers, finding openings)
• [x] Check detecting game being won when all sub-cells contain mines but large cell isn't split
• [x] Ensure number of mines is zero when changing difficulty in create mode
• [x] Fix bug with switching modes with odd dimensions

[codecov[bot]]

Codecov Report

Merging #155 (0e23374) into dev (4c1bf1a) will decrease coverage by 4.17%. The diff coverage is 63.44%.

@@            Coverage Diff             @@
##              dev     #155      +/-   ##
==========================================
- Coverage   74.89%   70.71%   -4.18%     
==========================================
  Files          29       38       +9     
  Lines        3151     3719     +568     
==========================================
+ Hits         2360     2630     +270     
- Misses        791     1089     +298     

Impacted Files	Coverage Δ
minegauler/core/split_cell/game.py	19.59% <19.59%> (ø)
minegauler/core/split_cell/minefield.py	24.00% <24.00%> (ø)
minegauler/core/split_cell/controller.py	31.64% <31.64%> (ø)
minegauler/core/engine.py	81.65% <32.14%> (-17.15%)	:arrow_down:
minegauler/frontend/minefield/split_cell.py	36.95% <36.95%> (ø)
minegauler/shared/highscores.py	70.04% <42.85%> (-1.00%)	:arrow_down:
minegauler/shared/utils.py	81.11% <50.00%> (-0.45%)	:arrow_down:
minegauler/frontend/main_window.py	51.14% <52.77%> (+0.02%)	:arrow_up:
minegauler/core/api.py	90.36% <70.58%> (-1.75%)	:arrow_down:
minegauler/core/split_cell/types.py	73.91% <73.91%> (ø)
... and 19 more

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