PreferencesHelper does not exclude events from being set on Preferences

Currently if a PreferencesHelper has an event trait and if the event has been fired, the value is also saved in the preferences.

I am not sure this is a desirable behaviour.

e.g.
```
from apptools.preferences.api import Preferences, PreferencesHelper
from traits.api import Event, Str

class MyPreferencesHelper(PreferencesHelper):
preferences_path = Str('my_section')

an_event = Event()

pref = Preferences(filename="tmp.ini")
helper = MyPreferencesHelper(preferences=pref)
helper.an_event = 1
pref.save()

```

Will create output like this:
```
[my_section]
an_event = 1
```

Expected no output.