Add option for merging local highscores

[LewisGaul]

Primarily needed over upgrade/reinstall, until #113 is fixed.

• [x] Pop up window to select highscores file(/dir)
• [x] Include version in SQLite highscores?
• [x] Ability to read in SQLite highscores
• [X] Ability to read in legacy highscores (deferred to #121)
• [x] Handle trying to read in the app's highscores (user error case)
• [x] Allow selecting the directory of another install
• [x] Ensure db is unaffected when no highscores are loaded (attached db should be detached)