SimC version option for character sims

Character sims now take a SimulationCraft version. A new SimC build sometimes carries a bug in one spec module, and switching versions is the way past it without waiting for the fix.

The option is under Advanced Options, in the SimC selector:

• Latest is the newest build available, and the right choice for most sims.
• Nightly is a build from yesterday or earlier.
• Weekly is a build that stays fixed for the week.