Rework state required for signal manager

[timwoj]

Once #39 is merged, we need to revisit the state required for the signal manager code. The changes for Windows necessitated moving the signal manager into the global namespace, which isn't ideal since it's being initialized and held in main.cc. This should be reworked to be more sensible.

From the PR:

We should rework the signal manager code to not hold global state in main.cc. If we need global state for Windows, then it should remain limited to the Windows CC file.