Properly integrate IMediaImporter methods

[Montellese]

IMediaImporter provides a lot of methods to query a specific media importer implementation about its capabilities and state. But some of them are currently not used because it is rather expensive to call them every time:

• [x] IsSourceReady()
• [ ] properly integrate IsImportReady() (may need caching)
• [x] get rid of CanUpdateMetadataOnSource()
• [x] combine CanUpdatePlaycountOnSource(), CanUpdateLastPlayedOnSource() and CanUpdateResumePositionOnSource() into CanUpdatePlaybackMetadataOnSource() and properly integrate it (may need caching)