Add long press activation for shutter lock

[gkoh]

Shutter lock currently requires 2 hands:

• hold button b
• press button a

This seems overly complicated.

This can be simplified to be just be:

• long press button a

Where 'long press' we can arbitrarily define as 'hold button a down for longer than 500ms'.

[gkoh]

Rejected as it may interfere with normal shutter release use.