Add support for window can become key

[thibaut-jarosz]

This allows popvover to be shown without main window resigning key.

[Arcnor]

Is there a reason this hasn't been merged yet?

[dbrisinda]

It seems to me that this line is somewhat problematic as it requires additional setup:

if ( _windowCanBecomeKey )
    [positionWindow makeKeyAndOrderFront:nil];

I found another way for document-based apps that use many windows. For example, clicking another document window, which will close the popover, and allow the newly clicked document window to remain key:

if (positionWindow.mainWindow)
    [positionWindow makeKeyAndOrderFront:nil];