Implement ProxyModel.

- remove 'view' alias property from Collection.
- add support for regular expression filtering.
- implement basic sorting function for sorting music
  as follow: artist > !year > album > track
- provide properties to access the proxy's filter and sort columns.
- add 'sectionMode' property to switch between views (accessible from
  the 'view' > 'presentation' menu.
Show first album section in sectionMode 3.