On OSX, program should not quit when last window is closed until user clicks away.

In order to keep the behaviour consistent with other programs on OSX, Arduino should not automatically close when the last window is closed, until the user clicks away. This allows the user the opportunity to open a new file, or change settings. Other programs such as QuickTime behave in this way.

[cmaglie]

Duplicate of #936

I'll add a note about how QuickTime behaves to the other issue, thanks.