Bug#429610: gbrowse2 licensing issue fixed

[Olivier Sallou <olivier.sallou@irisa.fr>]

I could have a look for dropdown css.
License is GNU GPL.
It comes from http://lwis.net/free-css-drop-down-menu/

For readseq.c, I decided to remove contrib directory from source, while 
other contribs are not clear from license point of view.
Contribs are not required for software usage.

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