frontend: Hold spacebar for arm/disarm funcitonality

[vshie]

Current behaviour

The slider to arm/disarm the vehicle is a pain, especially in the field when fingers are often damp on a trackpad!!

Expected or desired behaviour

Holding the spacebar, with Cockpit in focus to arm / disarm the vehicle would be much easier. Holding for 3-5 seconds should be long enough to not be triggered accidentally. This feature should be included in the cockpit tour, or mentioned when using the normal slider by mouse to inform the user of the approach. This spacebar approach matches newer versions of QGC!

Prerequisites

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