Added support for current_window setter.

[proneon267]

An API for setting a window into active focus was added.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

The general shape of this makes sense; I'm pretty sure the cocoa implementation shouldn't be much more than window.native.makeKeyAndFront().

However, the piece that is definitely missing is a test of this behavior. Ideally, this would be an automated test; but at the very least, we need a mechanism to exercise this functionality to demonstrate that it works.

The easiest way I can think of to test this would be an update to the examples/window app. There's a test action that creates multiple sub-windows; adding an extra action that will cycle the current window between the sub-windows a couple of times, and will also report them current window to the label in the main window.

[freakboy3742]

Thanks - I've made a couple more tweaks (one to correct the specific API invocation on Cocoa, and one to make the GUI test more visible); but otherwise, this looks good to go.

[proneon267]

Thanks