GridItem can be moved without being shown

GridItem can be moved without being shown; This also means the GridItem captures mouse events and changes the mouse cursor.

I propose that Items, which are not visible, should not be changeable.

Low effort:

Change the two occurences of if(!mControlWidget->getCalibGridFix()) to if(!mControlWidget->getCalibGridFix() && mControlWidget->getCalibGridShow())