Improvements and some bug fix

- bug fix taskbar buttons state;
- added tray song notifications;
- added tray menu shuffle command;
- added tray menu repeat command;
- added tray menu mute command;
- changed state of the taskbar buttons when click on the play icon in
playlists view.
Merge pull request #17 from Mishu/master

Taskbar buttons bug fix and minor improvements from Mishu