linux-v4l2: Make timestamp offset local.

This replaces the var in the source struct that are handling the
timestamp offset with a local one in the capture thread.
This change is mostly to make the code more readable.
linux-v4l2: Make frame counter local.

This replaces the var in the source struct that was used to print out
the number of captured frames with a local one.
linux-v4l2: Always display currently selected device.

This adds some code to the device enumeration that checks if the
currently selected device is present. In case it is not it will
add the device but disable it.
linux-v4l2: Disable properties when device is not there.

This adds checks to the properties callbacks that will disable those
properties when the device can not be opened to enumerate them.
linux-v4l2: Update properties when devices change.

This causes the properties for v4l2 sources to be refreshed when a
device is disconnected/connected.