Overhaul production hotkeys

This changes the sidebar production hotkeys to a configurable slot-based assignment as used in C&C3 and RA3 (the earlier games didn't have production hotkeys).

The default hotkeys are set to the F-keys (as C&C3 and RA3 did), which is a safe but non-optimal choice.  Players can adjust them in the settings menu to whatever they wish.

Fixes #3915, #4142, #4181, #6031.
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