[Icecat] Fix for 'Customize Mode' theme manager

After being using Icecat for some weeks, I realized it had a minor
problem when the time came to change its theme.

As much of you would know, Icecat, as well as Firefox, has a special menu
called 'Customize
Mode'. If you click it, you have the possibility to customize (wow!)
some aspects, such as toolbar location, icons, options... but also
themes.

In the lower left section of the screen, you can see a menu called
'Themes'. If you click it, you will be shown some basic 'sample' themes,
and a button called 'Get More Themes'.

If you click on it, you will be redirected to Firefox Addons website,
Icecat theme section... which does not exist. I wanted to replace that
broken URL, so I set to work.

I tried to grep a part of the broken URL inside Icecat source code, but
I didn't succeed. Finally, downloaded Firefox source code, and found
something useful: a .js file with some preferences, the one involving
the URL included. ([your firefox package].../browser/app/firefox.js)

After finding its 'Icecat substitute', I just copied the line of code
from Firefox to Icecat and voilà! It worked. I just want to share it
with you, as I think it should be fixed.

You have to head to /etc/icecat/syspref.js, and add the following line I
got from Firefox:

pref("lightweightThemes.getMoreURL",
"https://addons.mozilla.org/%LOCALE%/firefox/themes";);

This will redirect you to Firefox Themes whenever you click on 'Get More
Themes', but you can change the URL to whatever you like. For example,
there is another theme repo in gnu.org (check
http://www.gnu.org/software/gnuzilla/addons.html#themes), which is
contained in an interesting addon repository.

I hope this would help to make Gnuzilla a better browser, although I
know it's a minor change. However, big things start with small things!

I'm attaching some info links in case you can check my code (I'm
uploading this fix to GitHub) or follow my projects.

http://www.github.com/vlaze -- Github (profile still in beta)
http://www.twitter.com/xvlaze -- Twitter