user-panel: Don't wait for fprintd on initialization

When opening the user panel we g-c-c performs lots of sync operations that
may cause a noticeable slowdown, especially when a fingerprint device is
available, in fact set_fingerprint_label() call leads to:
  - DBus sync request of the system bus
  - fprintd dbus-activation
    + This leads to sync opening of all the devices, that might also cause
      a slowdown, depending on the devices drivers
  - Dbus sync calls to the device to get the list of enrolled fingerprints

Only after we've a reply, we update the g-c-c UI and continue the execution.

The fingerprint dialog code would need some global refactor, but to fix this
without big changes, let's just use GTask that runs a thread in wich we do
all the sync operations, and once done we finally update the widget state.