New Feature: Train units in batches when Shift-clicking

New Feature: When training units in a Town, building weapons in a War Factory or building ships in the Harbor, holding Shift while clicking on the item will now queue the unit in batches (8 for towns, 10 for war factories and 5 for harbours).

The numbers were chosen by the following arguments:
- For towns you often want to train a full regiment, which is 8.
- For war factories, the max. build queue is 20, so half-way (10) is a nice value
- For harbours, the max. build queue is 10, so half-way (5) is a nice value.