Implement actions/presets to control the OSD camera menu

[jswalden]

This enables implementation of the OSD Open/Close button, the OSD Enter button, and the OSD Back button on a PT-JOY-G4 -- plus up/down/left/right actions corresponding to what the joystick does when the menu (or any submenu of it) is open.

I have this implemented in a rev atop 833288e, which is also the branch-off point for a new release devoted specifically (and solely) to #33 and all its complexity. This will end up in the next release after that. IMO there's no reason to rush this into a release when every one of these buttons/actions is susceptible to being implemented using a "Custom command" action. (Although "OSD Open" technically requires two, to first close then toggle -- and that approach suboptimally will exit a submenu state to return to the main menu, unlike the implementation in this patch which sends a VISCA inquiry to avoid doing anything if the menu's already open.)