Allow assigning multiple keybinds to the same command

**Describe the feature**
Allow assigning multiple keybinds to the same command in the Key Bindings menu. Right now each command must have either one or zero keybinds assigned to it, but there are use cases where more than one would be preferable, similar to how most IDEs implement keybinds.

**Expected behavior and example use case**

For example, if you have select nearest idle engineer on 2 for general use cases:

this works well if you want to quickly select multiple engies and give a construction orders to them.

If you want to quickly give an attack move order though, you have to press and release alt between each selection, making the process much more error prone. For quickly giving attack move orders, putting the select nearest idle engineer hotkey on alt-2 would be better:

but if you do that you now have to _realease_ alt when you want to quickly give construction orders, making it unnecessarily fiddly.

The best solution would be to allow assigning both 2 and alt-2 to the same hotkey, so that you can use the appropriate keybind when it makes sense, e.g: something like this: