Improve the way how QtIvi loads it's backends

Currently QtIvi doesn't load plugins which doesn't match the current build configuration. Which means it doesn't load a debug plugin for applications in release mode. This is for stability reasons as mixing the configuration can lead to unforeseeable problems on some platforms.

The problem is that on mac where the configuration is decided on runtime, this can lead to the situation where no plugins are loaded at all. This needs to be improved.