#172 Added isIdling state to the SelectionHandler

[oleksandr-danylchenko]

Issue

[oleksandr-danylchenko]

@rsimon, here's an example of how the isIdling state can look like for the SelectionHandler. I intentionally chose to store just a single flag instead of some kind of enum, because that's the only state we're interested in for now. So it's more direct and takes less maintenance effort.

[oleksandr-danylchenko]

Another question is how to distribute that value to the selection state consumer. The Selection state is handled on the A9S side and shared via the Annotorious provider. So I think that one of the options would be to add the isIdling state there, but populate it only from within the R6O SelectionHandler for now.