frontend: main menu: simplify button labels

[ES-Alexander]

Current behaviour

Some button labels have two words, which requires more text space, results in some slightly weird spacing differences, and currently seems unnecessary.

Expected or desired behaviour

Per this PR review comment, I think it would be preferable to change the following labels:

"Edit Interface" --> "Layout" "Mission Planning" --> "Missions" "Enter Fullscreen" / "Exit Fullscreen" --> "Fullscreen" / "Windowed"

Prerequisites

• [x] I have checked to make sure that a similar request has not already been filed or fixed.