Make variables of DFB selectable for HMI value

[JeroenVH]

Currently it is only possible to select the whole DFB as an HMI variable. I would like to only select a single public DDT variable called "HMI" to be made visible.

The main reason for this is the limitation on variables in the NUA card. I have an array that is connected to an in/out of every DFB instance and this array is counted as HMI variable every instance. This way the limit of 100k variables is reached almost immediately. 

Schneider already suggested to move the whole HMI struct to an inout and make the only the connected variable an HMI variable. In my opinion this is a workaround that creates more work and goes against the good practice of reducing the amount of global variables.

It also makes it more likely to have a copy/paste error when you forget to change the inout when copying a DFB.