Remaining items for sorting out highscores

[XilefG]

• [x] Handle drag select changing [mid-game]
• [ ] Achieve reasonable test coverage (at least the new backend code)
• [ ] Show settings corresponding to displayed highscores in highscore window
• [ ] Update displayed highscores when a new one is set (call get_highscores() again - add a 'refresh' button?)
• [x] Keep highscore window state when it is closed (e.g. the name filter)
• [x] Do some manual performance/scale testing
• [ ] Add compatibility layer for legacy DB files and convert to current DB structure
• [ ] Handle opening highscores window when a custom difficulty is in play

Originally posted by @LewisGaul in https://github.com/LewisGaul/minegauler/pull/37#issuecomment-569116281

[LewisGaul]

Has been seen to perform very poorly with ~100 highscores shown at once.

[LewisGaul]

Closing in favour of #90.