Editor: ability to resize dual pane view

[gravelfreeman]

Add a small resize handle between the two panes so users can drag it left or right to resize both panes at the same time. This would make it easy to give more space to either the left or the right pane, depending on what the user wants to focus on.

The Width icon that currently disappears in dual pane mode could be a switch between :

• 50% - 50%
• 40% - 60%
• 60% - 40%