Enable floating panes. These are panes which sit above the layout
("tiled panes") like popups but unlike popups are not modal and behave
like panes (so the same escape sequence support). Floating panes are
created with the "new-pane" command, bound to * by default.
Currently floating panes can only be moved and resized using the mouse.
The default second status line (if "status-format" is set to 2) has
changed to show a list of panes.
This is the first step and many obvious features are not yet complete
(notably the ability to swap floating panes, resize them using
resize-pane, change them between floating and tiles, and restore custom
layouts with floating panes).
Mostly written by Michael Grant with help from Dane Jensen.