Multiple windows support

[mvlabat]

Based on #13

Issues to fix:

• When a new window is spawned, it acquires the focus, at least on Windows (TODO: test other platforms), but the active context is still the default window. That results in events coming from the second window being consumed by the default context.
• Clipboard can paste into several fields at the same time if they are all active in different windows.

[mvlabat]

@jakobhellermann I've extended the two windows example a bit and found a couple of minor bugs. I'm looking into them right now, but if you already have an idea on how to fix those, that will help. :)

[jakobhellermann]

@jakobhellermann I've extended the two windows example a bit and found a couple of minor bugs. I'm looking into them right now, but if you already have an idea on how to fix those, that will help. :)

What bugs are there? Running locally seems to work for me.

[mvlabat]

@jakobhellermann sorry, I forgot to mention that I've described them in the PR body. :) The clipboard issue is now fixed, now I'm trying to figure out what to do with the invalid initial focus.

[jakobhellermann]

On linux the focus doesn't seem to be an issue. The second window gets focused as well, but tab presses etc. get correctly handled in the second window.

[mvlabat]

Yeah, it looks Windows specific... I wasn't able to reproduce it on my Macbook either.