All the plug-ins were updated to support the new vision on plugings in Qt5

Old style plug-ins required a macro at the end of the source file:
```
#if QT_VERSION >= 0x050000
Q_EXPORT_PLUGIN2(lc_sample, LC_Sample);
#endif
```

This is just a stub in Qt5 and produces a static assertion. The moc tool is used to generate the plug-ins so the definition has been moved to the header:
```
#if QT_VERSION >= 0x050000
     Q_PLUGIN_METADATA(IID "org.librecad.sample" FILE  "sample.json")
#endif
```

A .json file needs to be added and it specifies the keys that will be part of the executable binary.

References:
[The Transition from Qt 4.x to Qt 5](http://qt-project.org/wiki/Transition_from_Qt_4.x_to_Qt5)