Implement window focusing

[leolost2605]

When an application has more than one window open show a window list in the context menu and focus the window when the entry is clicked on. The icon doesn't show up for me even though it's set via MenuItem.set_icon do I have to do something else here?

Requires elementary/gala#1817

[danirabbit]

Gonna push this back to draft for now since it has lots of conflicts and we have the window spread and icon scrolling merged in for switching between multiple instances. There wasn't a clear desire for showing windows as menuitems in the survey, so I think it's worth seeing how it plays out with the other multitasking methods and see if we really need this too