Improve Python access to ctkSettingsPanel

Add a Q_INVOKABLE overload of ctkSettingsPanel::registerProperty in
order to make the method usable from PythonQt. The overload takes the
signal as a QByteArray, to match the behavior of qt.SIGNAL, so that
users do not need to explicitly cast that to a str in order to call the
method. Also expose settings/setSettings as a Q_PROPERTY, for similar
reasons.
Improve Python access to ctkSettingsDialog

Make addPanel methods Q_INVOKABLE to allow use via PythonQt. Make
various other attributes accessible as Q_PROPERTY's. Improve
documentation.
Merge pull request #453 from mwoehlke-kitware/ctksettingspanel-python

Make ctkSettings{Dialog,Panel} usable via PythonQt