keyboard: Add "Alternate Characters Key" configuration

This adds an entry to select the "Level 3" selection key, which is
usually Right Alt (or Alt-Gr on a majority of keyboards).
Mockups at:
https://raw.githubusercontent.com/gnome-design-team/gnome-mockups/master/system-settings/keyboard/keyboard-wires.png

However, we replaced the "Left Ctrl" key option with a "Menu key"
option, as "Left Ctrl" isn't a possible XKB option for the level3 key.
11 jobs for wip/alt-chars-key in 4 minutes and 47 seconds (queued for 3 seconds)
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