libtorc-media: Refactor much of libtorc-media

- add TorcMediaMaster, which acts as the master list of known media of
all types.
- refactor TorcMedia to make it a QObject with properties that can be
retrieved.
- add TorcMediaDescription, a 'proxy' for TorcMedia that can be passed
around.
- update TorcMediaSourceDirectory to actually pass on details of new and
removed files for the directories it is monitoring.
torc-desktop: Add view of known media files