Updating look of CK2OpenPanel for 10.10+

[MrNoodle]

How do you guys feel about me updating the look of the open panel to be more in line with NSOpenPanel on 10.10 and later?

The main thing would be the lack of titlebar. I'm not sure how we would incorporate the header we currently use (the one with the machine name) but I'm open to eliminating that. The same info can be found in the path control/pop-up.

I think it can be done without breaking compatibility with older OS versions by checking method availability at runtime.