Ability to add units to or remove from an already selected group of units

[Kieran P]

The game needs a way to select units to add to a group of already selected units.

All of the following happen when holding the SHIFT key:

• Clicking a single unselected unit should add them to the group of already selected units

• Clicking a single selected unit should remove them from the group of already selected units

• Dragging over a group units where any/all of them in the dragzone are unselected should select the unselected ones, while keeping the selected ones selected

• Dragging over a group of units where all of them in the dragzone are selected should remove them all from the group.

The latter two options should show the dragging zone as normal, and use the unit selection rings to indicate what will happen. e.g. for the last one, unselecting a group of units should remove the unit selection ring as you dragzone over them.

[Kieran P]

I've attached a patch to get SHIFT+Click and SHIFT+Drag adding units to an existing group of selected units.

After a bit of thought, I decided only to implement points 1 and 3 in the original ticket. Having SHIFT+Drag unselect units in certain cases would be inconsistant.

[fcxSanya]

Patch waits for a review, right?

[brian]

(In [8332]) Made EntityGroups more efficient

"Ability to add units to or remove from an already selected group of units" based on patch by k776 (modified to avoid going over the max selection size) (Fixes #602)

[brian]

reviewed