Monday maintenance

This is a bugfix patch that fixes some more issues that cropped up after the update:

• Holding the Line: There was a crash bug in this mission, triggered by a rare and specific shapes of the growing rift. This is now fixed.
• Ruin events vs probes: Launching a probe, then quickly opening an event before the probe launches its pulse would put the undo stack in a badly ordered state, preventing undo when it should be available. This would also cause crashes when cancelling actions during two of the events (Planet Crusher and Terraformer).
• Performance: Stat-tracking (counting eg. colonized planets for achievements purposes) would sometimes cause a small lag after an action. This should no longer be the case.

This patch brings the version numbers up to v1.2 (b877) on Windows and v1.2 (b878) on Mac.