There is one problem with this, I need to know beforehand what Emacs
is going to to with the URL to tell the drop source if Emacs is
accepting
the URL or not. Consider dropping text that will be inserted in a
buffer.
It is not useful (or easy to make it work correctly) to drop that on
the
tool bar, menu bar or scroll bar. If you have multiple windows in the
frame, it is hard to select the one the user intended. Also, if the
buffer is read only, we don't want to accept a drop there. So
currently
these kind of drops are not accepted.