Add support for named, task-related groups of windows

[mforkel]

I usually work on multiple tasks, often using groups of Firefox windows for each of them. Because I have found no easy way to save all windows belonging to one task when I stop working on that task for a while, I end up with many minimized windows that consume memory and make restarts slow. Also, it is hard to identify all the windows belonging to one task once I start working on it again.

For me it would be very helpful if I could manage named, task-related groups of Firefox windows. Typical operations would include assigning a new window to a new or existing task group, opening and closing all windows belonging to a task group, and removing a windows from a task (implicit when closing a window). It should be possible to have multiple groups of windows open at the same time. The name of the group a windows belongs to should be displayed in the window.

Helpful operations on the windows of a task group would be exporting and then importing them on a different computer or being able to move all of them to a different virtual desktop.