Add OnPasteClipboard method to CGUIDialogKeyboardGeneric

This is a follow up to https://github.com/xbmc/xbmc/pull/3026. It adds a handler for ACTION_PASTE to CGUIDialogKeyboardGeneric.

I've duplicated some code from CGUIEditControl. If this gets the nod it would be tidier to put the common code in the utils somewhere. I'll need advice on the best place to put this code.