smartcard: check for the addition of new smartcard readers

gsd-smartcard currently checks for the insertion/removal of smartcards in reader devices which where present at the time gsd-smartcard was started, but does not account for new smartcard readers appearing after gsd-smartcard was started.

This patch adds support for checking for the addition of new slots (i.e. smartcard readers), which is necessary to support devices like the Yubikey (a "reader" and a smartcard) which may be inserted after gsd-smartcard was started.