Instead we define the class with G_DEFINE_TYPE_WITH_PRIVATE() or
include G_ADD_PRIVATE() when defining with code.

This also removes the usage of G_TYPE_INSTANCE_GET_PRIVATE() as we use
instead the _get_instance_private()

Notes:
- Only plugin skipped is Tracker as it needs more adjustments
- Some Private structures had to be renamed
  e.g: GrlLocalMetadataSourcePriv -> GrlLocalMetadataSourcePrivate
  e.g: GrlMetadataStorePrivate -> GrlMetadataStoreSourcePrivate
- The G_DEFINE_TYPE_WITH_PRIVATE() declaration might be moved as it
  must be after the private struct definition and before the call for
  _get_instance_private()
- Tested with grilo-test-ui and totem