QGCCameraControl::storageFreeStr(): use QGCMapEngine::storageFreeSizeToString()

Fix merge screwup

Support up to 64 buttons, not limit to 16
Send a single button command and not a stream of commands at 25Hz
Send button commands through signals (it's coming from a different thread)
Hide gimbal support through analog channels (there are no MAVLink messages to handle it)

Map gimbal controls
Keep track of gimbal position within Vehicle

Add gimbal to external control signal

Guard against no geometry being set

Add camera trigger
Add video start/stop/toggle recording
Add gimbal pitch and yaw