Add support for time-limited actor upgrades.

This adds support for limited-time upgrades, and adds a warheads and support power upgrade source.

These changes are demonstrated by porting the Iron Curtain to use upgrades, adding an invulnerability crate to RA, and implementing the plumbing for EMP in TS (but doesn't attempt to fix the other limitations).
This causes problems if two actors grant the same upgrade, and then one removes it.  I will need to keep track of individual upgrade events so that stacking works correctly.