Context menu for release table view and related change/improvements

- Implemented a context menu for table view component
- CellFactory class is retrofitted to support multiple cell customizers
- Fixed a bug when loading a release without artwork caused NPE
- Tooltips are temporarily removed from release URL column since they
were causing problems when switching to a browser window
- "Play Selected..." button was removed because this action now appears
in the context menu
- Various small improvements