Don't show wallet options in the preferences menu when running with -dis...

See https://github.com/bitcoin/bitcoin/pull/5222


Is it worth to have a initializer parameter (enableWallet) for this?
Would it not be possible to wrap the bool through the OptionsModel instance?
My idea would be to do...
   ui->tabWidget->removeTab(...)
... when setting the optionsModel and after checking thought the optionsModel if the wallet is enabled.