feat(hotkey): Enable players to hold down hotkeys to queue multiple units

[Caball009]

QoL improvement to make it more convenient to add units to build queues by holding down hotkeys.

Currently, a hotkey needs to be held down for longer than ~125 milliseconds to start queuing multiple units. Next units will take just over ~40 milliseconds (1 / 3 of the initial delay) to queue.

[Skaronator]

It would be really cool if holding multiple hotkeys at once would queue units alternatingly between them.

Example: holding A and B could queue units in an A → B → A → B pattern.

[Skaronator]

This feature would also be really useful if you want to build a unit, but you are out of money. So you just hold down the button and no longer need to spam it.

[Skyaero42]

What is the risk that a player accidentally builds one extra unit due to holding the key too long?
This feature may need to be behind an options.ini setting.

[Caball009]

This feature may need to be behind an options.ini setting.

I think that's fair. I'll look into that.

[Caball009]

It would be really cool if holding multiple hotkeys at once would queue units alternatingly between them.

Example: holding A and B could queue units in an A → B → A → B pattern.

I'll check whether this is feasible.

This feature would also be really useful if you want to build a unit, but you are out of money. So you just hold down the button and no longer need to spam it.

Being able to queue units without paying in advance is not related to this PR and requires a bigger change.