Move frame saving into its own thread. Fixes #6.

This prevents the UI from locking up at high framerates.
split process_framedata into save/preview.
Improve frame manager logging.
Rate-limit preview updates (defaults to 500ms).
Modify flat/dark countdown into a general burst mode. Fixes #3.
Limit simulated frame size hardcoding to one place.
Fix reduction command creation.
_WIN32/_WIN64 #define cleanup.

_WIN32 is defined on WIN64, and WIN64 definitions
should take priority.
Update windows install docs.
Update MSYS default for new install.
Extract set_float_param in camera_picam.
Commit ProEM temperature at startup. Fixes #4.
Add ProEM EM gain preference. Fixes #8.
Add ProEM row shift rate preference.