Provide layout option for "popped out" windows

[cmeyer]

Either separate floating windows (with no panels attached); or within the workspace... a floating layout.

User should be able to set the default layout of popped out windows.

This could be used for a dedicated "monitor" window.

State of the window should be persistent, like main window.

More notes:

• Associate monitor with either workspace or project
• Window opens automatically when switching to workspace or project
• Monitor controller class, like camera, so that monitor is generally useful
• Display panel overlay choices could be used for monitor, also could be used for slice interval

[cmeyer]

Part of DMH changes.

[cmeyer]

Also see https://i3wm.org/docs/userguide.html