Compile under Qt5.4. Please review the use of toUtf8 instead of toAscii

As we can install Qt 4 and 5 side by side and almost every distribution have the newer, even Debian Wheezy trough backports, I started to port Belle. With this minimal changes I can compile in 5.4.
But I'm not sure if `toAscii` can be changed to `toUtf8` without side effects to other part of code or if it should be `toLatin1`.
This class was needed because Qt4 didn't provide a way to read/write JSON documents. However in Qt5 there's a new class available exactly for this, so we should use it instead.