Allow reload in case of external changes

Add `ctkSettingsDialog::reloadSettings()` method to allow reloading settings in case of external changes. Rename / make `public` the `ctkSettingsPanel::updateProperties()` method (to `reloadSettings()` also) in order to implement this.

This provides a way to make the settings dialog / panels aware of changes made to the relevant [QSettings](http://qt-project.org/doc/qt-4.8/qsettings.html) outside of the dialog, which is both an obvious bug in its own right, but more importantly prevents the dialog from overwriting such external changes (see commontk/CTK#450).