Fixes #6749 and related to #7006.
This slightly refactors the way how setting a Proxy over the UI settings panel works. It extends and used the QValidatedLineInput class.
Two goals of this PRs:
- Setting a invalid proxy should no longer be possible (which could lead to a startup termination)
- An Invalid proxy get the "red" invalid input-field background (similar like an invalid bitcoin address in the pay-to-address input-field)
<img width="704" alt="bildschirmfoto 2015-11-16 um 11 01 17" src="https://cloud.githubusercontent.com/assets/178464/11179265/6f5e6ad4-8c52-11e5-9597-aad2a46510a4.png">
