String change in Brasero

[Philippe Rouquier <bonfire-app wanadoo fr>]

 Hi all,

I had to change two strings in brasero to fix bug #570235. I changed the two strings:
"Substitute" and "by"
into
"_Replace" and "with".

First it avoids problems with the above bug (though contexts could have been used as well).
Then, it also correct a grammar issue since it is "substitute A _for_ B". Moreover "substitute A for B" means B is changed into A which was the opposite in the UI.
That was also an opportunity to add a mnemonic which was missing.

Here is the change:
http://svn.gnome.org/viewvc/brasero?view=revision&revision=1887

Thanks,

Philippe