Split some options in to `GeneralTab_LocalisationGroup`

[originalfoo]

Part of #1356 Phase 2.

• Localisation group of options now in its own class
• Also change mph option to CheckboxOption
• Will need testing at main menu (they are global options) and also in-game

Mod options which need testing:

• Lanugage selection
• MPH speeds option
• Speed limit icon theme

Additionally:

• Toggling mph button on speed limits palette in-game

Remaining groups will be handled via separate PRs.

Compiled mod for testing: TMPE.zip

[originalfoo]

ready for review

[originalfoo]

@krzychu124 any feedback on this PR?

[krzychu124]

@krzychu124 any feedback on this PR?

Looks ok, but I wanted to test in game (I was really absorbed with aircraft AI experiments yesterday 😄)

[originalfoo]

Looks ok, but I wanted to test in game (I was really absorbed with aircraft AI experiments yesterday 😄)

I've got another PR coming after this one that will complete work on the General tab groups so can test in there if preferred?