Add preferred position functionnality to shutter

[PoppyPop]

Hello,

In the control APP, i have access to a "preferred position" for my shutter. After some test and digging in the API, i noticed that there is an undocumented value of -2 for this.

Here is a small PR that add the functionnality.

i tested it in my setup and it's working well. i modified the code, to be backward compatible (low value return -1 as before for example)

Feel free to ask if you need more screenshot or proof of the functionnality

Have a nice day !

FYI, it's the round icon at the right