Implement Ctrl B - Cycle through idle construction units except Commander

[KevinHake]

Functionality of ~~OTA~~ demo recorder modified OTA: Ctrl-B searches for a build unit that is not the commander and is not busy. The search appears to be in the order the units were constructed (similar to track ['T'] command). If a unit is found, it is selected and the camera is centered on the unit (tracking not enabled, and no real need since the unit won't be moving anyway). Continued Ctrl-B will start the search from the unit last selected by Ctrl-B, regardless of what was selected at the moment Ctrl-B was pressed.

Need to decide if things like mobile nuke defense units should be included in Ctrl-B, or in Ctrl-F, or both... given they build via menus just like factories, but are mobile.

[Skirmisher]

This functionality was added by the demo recorder, fwiw. The original game just selected all non-commander construction units. Not that I think anyone misses that...