keyboard: Add "Alternate Characters Key" configuration

A couple of TODO items to resolve:

• The layout for the dialogue itself doesn't exactly match the mockups, but I didn't have the patience to finish this, especially as glade doesn't seem to show me something that resembles the dialogue once it's run
• Should the subtitle of the option be smaller? It's smaller in the mockups, but not smaller in other panels which use this pattern
• Should the subtitle finish in a period? The text in the Power panel uses a period, the mockup doesn't.