quicknanobrowser source code example not compatible with Qt < 5.13

• install via maintenance tool new Qt 5.13 version in addition to already existing 5.12
• go to examples and try compile quicknanobrowser example

observed:

• it start fine under 5.13.2
• it crash/abort on 5.12.4

The problem is with initialization; as documentation explain this line of code:

QtWebEngine::initialize(); 

have to be initialized before 'Application' in Qt 5.13.2 but after 'Application' in Qt 5.12.x.

There are missing some Qt version checks in source code examples.