Don't cancel attack move cursor when holding shift

[PunkPun]

Closes #16924

This how it works in SC2. While you hold down shift the attack cursor remains

[dragunoff]

I understand the rationale for this change and I tentatively agree that this is how it should work. Should guard order behave in the same way?

Also there is another problem with the a-move in OpenRA - once you enter a-move mode by pressing "A" there's no easy way to exit that mode from the keyboard (pressing "A" again doesn't toggle the mode). The only ways to exit the a-move mode are by issuing an order, by deselecting, by switching to another order mode (e.g. guard) or by double tapping Esc to open and close the menu. I bet SC2 has a way of exiting the order mode (maybe right click or Esc).

[PunkPun]

Since attack move is a mouse only order so I doubt that it has many other ways of cancelation. But this is unrelated to this PR

Should guard order behave in the same way?

It could be considered. Now that I think about it more people may want to put guard order on multiple units

[PunkPun]

Adding it to guard won't be as simple and since this PR already has +2 it's better served in another PR

[abcdefg30]

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