Implement batch-training

[andy5995]

When a single building is selected, or multiple, there should be an option to "batch-train". Something like "shift-click" will give the command to train more than 1 unit at a time from each barracks.

[andy5995]

For a more specific example..

As Indian, If I have 1 round tent selected, pressing some modifier key and then clicking the unit I want to produce would produce more of them than just one.

The benefit would be not having to click for each unit produced.

If I have 3 round tents selected, and batch-training is set for "5" units, pressing a modifier key and then clicking once on a fire archer would produce 15 fire archers.

The way it's implemented in 0 A.D.

When I have a building selected, and I press "Shift", the number of units that would be batch-trained is overlaid on the units.. it applies to all units, but if you don't have the cash, the option to produce is disabled. So.. a little 5 gets put over all the icons for the trainable units, for example.

The number can be modified by using the mouse-wheel while holding the shift-key. Then you click once to train that number.

The initial number for batch training is set from the game options.

[andy5995]

related screenshot example at https://github.com/ZetaGlest/zetaglest-source/issues/245#issuecomment-425241707