Split panes

[hojberg]

Allow the user to open a new split pane and to move open definitions within that pane, move freely between panes and move definitions between panes. The cursor/focused definition within each pane should persist.

Open questions:

• Should there be unlimited number of panes?
• Should we allow both horizontal and vertical panes?
• Are there an alternative to panes to solve the same problem, like popping out a definition in a temporary sticky window?