Multiple track panels support.

[iromeo]

At the moment, JBR supports only a single panel with scroll pane & tracks. We need a support of multiple panels

Requirements:

• [x] Individual vertical scroll support in each panel
• [x] Do not render tracks that are not visible in any of the panels (decided not to implement at the moment)
• [x] Mouse actions support (track context menu, information under cursor, on click)
• [x] Allow moving panels
• [x] Allow moving tracks inside panels and between panels
• [x] Allow to create panel from group of selected tracks
• [x] Allow to delete panel and 1) keep track, put them e.g. in previous panel 2) also delete tracks.

Subtasks:

• [x] Subtask: https://github.com/JetBrains-Research/jbr/issues/248
• [ ] Subtask: https://github.com/JetBrains-Research/jbr/issues/249
• [x] Subtask: https://github.com/JetBrains-Research/jbr/issues/253
• [ ] Subtask: https://github.com/JetBrains-Research/jbr/issues/263

[iromeo]

At the moment, panel size is a % of browser height and not a fixed size in pixels. When user changes browser height all panels will be resized.