Update (and cosmetic fixes) for 'pakfire.cgi': Added GUI entry for existing, but...

Update (and cosmetic fixes) for 'pakfire.cgi': Added GUI entry for existing, but unused translation string

While preparing the Core153 update, I found by chance that a language string had been added from
Core152 to Core153 which I couldn't find in any CGI-file.

The translation suggested that this string ('Available Updates') could belong to 'pakfire.cgi'.

And I thought that on the pakfire GUI something was actually missing: the heading above the
box listing the 'Available Updates'. Don't know why I didn't saw this before.

So tried to add these missing heading. I hope I made it right...

Some cosmetic fixes:
I also added some space around the text for 'Available Addons' and 'Installed Addons'
because the text lines weren't separated. There is no seen wordwrapping. This required deleting
some unwanted '<br />' in the affected translation strings.

I tried this about 4 years ago, but somehow this patch got lost.

Signed-off-by: Matthias Fischer <matthias.fischer@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>