Add a method to detect when the state-machine can exit.

According to the SCXML specification, the state-machine can exit the
interpreter loop when a final state has been reached. This happens after
finishing a macro-step or when it ends up in a final state right after
startup (e.g. when the only state is a final state). This patch adds a
virtual method which can be used to detect this reliably.

Change-Id: I594e952a4972dd70d4089a2c4ce1c86880568eb9
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>