Handle inactive monitors better

[jeremypw]

Fixes #446

This PR addresses a number of issues around toggling the enabled property of monitors.

• Inactive monitors no longer are drawn as part of the layout, but as a smaller widget on the side
• Layouts that include a disabled monitor have a different key
• After changing the active state of a monitor the menu pops down and the "Apply" button is made sensitive
• After changing the active state of a monitor the remaining displays are rescaled and redrawn as if the change has been applied

This heads towards #74 although the disabled displaywidgets are currently still drawn on the overlay. A future improvement would be to put them in a separate sidebar widget.

[leonardo-lemos]

Code looks good. Thanks.