Add next/prev buttons

[beshur]

Add buttons for next/previous track.
Interface investigation required.

[endway]

As a result of an investigation, I've come with the options below:

1. Bind hotkeys for prev/next track.
2. Create a popup, which shows mini-player with title, prev, pause, next buttons.

In order to leave single click functionality for play/pause we can only bind hotkeys for next/previous track.

[beshur]

Let's do the hotkeys for prev/next track