[New Feature] Prevent left click from deselecting already selected units

[Bittah]

First Check

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Description

One of the most frustrating things that surely happens to many players at least once every so often is to have some units selected and then attempting to quickly order them to move away from danger, only to accidentally click on one of your own units, which then selects that unit instead of giving the move order and causing you to lose the units that you meant to order to retreat.

A good solution for this would be to have the game disallow you from selecting a friendly unit/structure by left-clicking on it whenever you already have units selected; clicking on a friendly unit/structure should then instead give a force-move order so that your selected units will move to the nearest cells adjacent to the unit/structure you clicked on. I don't think that I ever intentionally select units by left-clicking on them before manually deselecting my already selected units in the first place, but in case anyone else might, this could be made into a game option.

Proposed Code (optional)

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Additional Context

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