bug#19500: 25.0.50; Non-t follow-link property doesn't work

[Dmitry Gutov]

Evaluate this:

(let ((beg (point)))
  (insert "foo")
  (add-text-properties beg (point)
                       '(follow-link [down])))

Then click on "foo". In Emacs 24.3, that would move point to the line
below it.

In emacs-24 and master, that just triggers paste (IOW, delegates to
mouse-2, which calls `mouse-yank-primary').

Whoever fixes this, please add a regression test.

In GNU Emacs 25.0.50.24 (x86_64-unknown-linux-gnu, GTK+ Version 3.12.2)
 of 2015-01-04 on axl
Repository revision: d94007484ab6b98054b043c1bedeaf7a58eb8606
Windowing system distributor `The X.Org Foundation', version 11.0.11601901
System Description:     Ubuntu 14.10