Implement C++ model (QAbstractTableModel).

- the C++ CollectionModel provide basic model/view capabilities.
  it handles role-names and provides an invockable get() method.
  for testing purpose, it also provides a mechanism to populate
  the collection from a JSON serialized file.
- using the CollectionItem delegate for saving properties such
  as title, artist, album is bad. the delegate is managed by
  the ListView and may not be initialized or already deleted.
- use only one font instead of two.
- add a sample music collection.
- add bold variant from muli font.