Track clients connected to each device using EggDbusMonitor.
When there are no more clients connected to the device, release it.
When no devices are used anymore, set up a timeout to exit within
30 seconds.
Make all of FPrintManager's struct members private,
warn of errors using a _get_error() function, as object
creation can never fail.
The only error possible shouldn't really be an error though,
as it only means that no devices will be enumerated, but it
could tell us about newly plugged devices instead.
