Clipboard paste fixes

This fixes #156 (closed). "Paste from clipboard" is a separate menu option now (shortcut is cmd/ctrl+alt+v). I also fixed some bugs and improved the error handling, so that pasting complete subpatches via the clipboard works now, and pasting incomplete ones doesn't make Pd crash any more, but yields an error message in the console. Details can be found in the individual commits. All in all this seems much more robust now, and I couldn't make it crash any more by feeding random blocks of lines from an existing .pd file.

@jwilkes I've tested this quite thoroughly on Linux, but it would be nice if you could also give it a whirl on OSX and Windows. In particular, please have a look at whether I did the additions to the Mac menu correctly, that was mostly guesswork. :) Thx!