Optimise saving profile shortcuts

- Call ProfileSettings::slotAccepted() when the parent ConfigurationDialog
  is accepted, so that ProfileManager::saveSettings() is called
- don't delete the ProfileSettings dialog on accept, it'll be destroyed when
  the parent ConfigurationDialog is destroyed
- save the profiles' shorcuts if they were actually changed

With this commit and the previous one, there is no need for ~Part() or
~Application() to call saveSettings(), shorcuts settings are saved when
the ProfileSettings dialog is accepted and saving the default profile is
done in setDefaultProfile(). This fixes an issue where changing e.g. the
default profile is only saved to the konsolerc file when the main window is
closed.