[grilo-plugins] tracker notification support v2

Hi all,

Just an update on the notification support, with some bugs fixed.

Regards,

--
Lionel Landwerlin

Follow-Ups:
- [PATCH 3/5] tracker: support notification without per device mode
  - From: lionel . g . landwerlin
- [PATCH 4/5] tracker: add missing copyrights
  - From: lionel . g . landwerlin
- [PATCH 5/5] tracker: fix notifications bugs
  - From: lionel . g . landwerlin
- [PATCH 2/5] tracker: add notification support
  - From: lionel . g . landwerlin
- [PATCH 1/5] tracker: switch to tracker:id as Grilo key id instead of URN
  - From: lionel . g . landwerlin