Use grey disabled icons for each wallet instead of the question mark ico...

A few bitcointalk users reported that they didn't think that the current hidden wallets were hidden wallets. To make this more obvious, someone on the Bitcoin Foundation forum suggested to use grey icons of each wallet. This pull request allows this.

Since each wallet name and link remains unavailable before the user clicks the warning, it seems like a good way to do it. Any more opinions?

![Capture du 2013-04-08 15:23:16](https://f.cloud.github.com/assets/3578089/353454/0e754148-a082-11e2-936f-ef97826123da.png)