Automatic inheritance (#29)

A wrapper trait automatically extends its Scaloid parent in
accordance with Android class hierarchy. If the parent wrapper not
exists, it raises compile error that helps avoid mistakes.

Related changes:

 1. richClassDef/wholeClassDef template has optional mixin argument for
   manual inheritance.

 2. Wrapped `android.preference.Preference` and
   `android.preference.DialogPreference` that are ancestors of
   `android.preference.EditTextPreference`.

 3. Renamed `defaultValue` to `_defaultValue` due to confliction with
   `android.preference.DialogPreference`, and made it private in Scaloid
   packages.


Change 3 messed up the diff, but no other parts of generated code than described above are changed.