Today, after installing the latest QT - which took forever! - I managed to get the grblController compiled on my system. It's actually very easy, just don't follow the build instructions. Installing the latest free QT is enough to build it, if you install the entire package including MinGW for your 5.9.x installation.
So after waiting for about 2h for the installation to finish I opened the project file and was presented with a nicely structured code in good old QT manner with signals and slots and the goodies I love about QT.
To my surprise it was easy to compile. There are two standard QT includes missing which I added into my fork of grblHoming (the actual name of grblController). After that was done, I started adding buttons and some text fields that I wanted as comfort functions for PCB milling.
As you can see in the image above, I added a new tab specifically for PCB milling. It offers an actual homing command ("$H") for grbl, probing commands using some parameters for the probing as well as setting the machine offset after homing for milling the bottom side. All of this works really well now and I'm eager to start some actual projects with the mill.
At the bottom, I also started to add some stuff for autoleveling. I think this is a very useful function for PCB milling since the milling depth is in the -0.1 mm to -0.2 mm range and every little deviation from Z=0 will cause your track quality to suffer dramatically. So, if I run into problems, I might add this feature later. Right now, the mill bed is flat and I don't have an immediate need to implement this into grblController.