Make CurrentWindows a generalized window container.

[dmwyatt]

CurrentWindows is currently structured in such a way that it only represents all windows on the system.

It would be better if it could represent any group of windows.

Potential API thoughts

• When keyed by window titles, this class returns a list of matching windows. Instead return another instance of CurrentWindows.
• Many of the methods and properties on the Window class could be mirrored onto the CurrentWindows class so you could, for example, minimize all of the represented windows, or move all of them to a specified monitor.
• Is it worth supporting indexing and slicing like the lists that we currently return? How would this work with keying by title?