Switched from QComboBox to DropLineEdit for pathgrid creator input.
This allows the input the use auto-complete and be a drop target from
the cells table.
When data changes the cell input for pathgrid creator is repopulated
with valid choices. This handles the case where a cell is added or
removed, and also when a cell's pathgrid is added or completely
removed.
Instead of using QLineEdit for user input, use a QComboBox populated
with valid choices. This prevents user from being able to create a
pathgrid for a non-existent cell.