Clipboard support

GUI applications often require access to the clipboard. Is this something you'd like to have implemented in libui or is that out of the scope of this project? On Linux, for example, we already link against GTK so we might as well make use of the clipboard API that it has. If it's not considered out of scope, I'll try to implement clipboard support and submit a PR. I'll need some help with the OSX part though as I don't own a mac and I don't speak Objective C.