Attached patch adds yank-media and DND handler to attach files in the
clipboard and dropped onto an Emacs frame respectively.

The yank-media handler for images is well tested, I use it frequently
however, rest of the stuff aren't really tested since I don't use a GUI
file manager.  I tested enough to make sure the logic is right so I
don't know if they are ergonomic enough.  As noted in the comments,
files copied/cut to clipboard doesn't seem to have a solid spec and most
of the code was written with pcmanfm as the file manager, testing with
other file managers are highly welcome.

Better names for the newly added defcustoms are highly welcome since I
don't find them to be clear enough from their name.

The patch compiles cleanly without warnings but I haven't tested _this_
patch yet, I have these functions in my init.el and have tested those.