Some UI stuff

There are some simple UI features that I hope are planned or could be included.

• Ability to select multiple factories at once. Mostly need for setting rally points efficiently.
• Ability to queue a factory production order as 'only build once, and move to front of production queue.' This is very useful for the case where you want to build a unit now, but without interrupting the build order for a factory on continuous build. For example, if your opponent gets bombers out, and you need just one Apollo at each area. Or you want your new factory to build 2 Hermes before it starts on its continuous build queue. 'Ctrl' works well as an escape key for this.
• A hotkey to cancel production at a factory.
• A hotkey to toggle continuous build.