Add very basic implementation of the flash clipboard

This is just a very simple implementation of the clipboard, it only makes it possible to copy and paste text via the gtk clipboard.
Just thought people might want this little bit of functionality, I'm not really sure what to do with the ppb_flash_clipboard_register_custom_format. @i-rinat any idea what I should do with those?

The ppb_flash_clipboard_is_format_available could just check what type it is and see if there is an equivalent to that in gtk, gtk has support for images and text afaik so that should be possible.