Adds a general warning to the Qt (Debug) Console regardless if the wallet is encrypted or not. The scope of this PR is also not to warn or disable certain private-key revealing commands (can be done in a separate PR).
Inspired by @laanwj's comment in... #8544 (comment)
<img width="852" alt="bildschirmfoto 2016-12-12 um 16 00 55" src="https://cloud.githubusercontent.com/assets/178464/21103802/36e36d00-c084-11e6-8d83-0407e9be8de1.png">