fix low-risk compiler warnings

Summary

There are a lot of compiler warnings (currently 81). Some of them may just need annotations (e.g. indicating with fallthrough is intentional in a case statement), others may be bugs that we just haven't happened to hit (casting from a char to a char*), and still others are probably just bad practice that don't have any functional impact (e.g. unused variables), but it'd be nice to clean them up so they aren't cluttering things up, potentially obscuring things that really are a problem.

This task is to fix as many of these as we can, as long as we are sure that they will not cause any regression bugs.

Note: This task is not scoped to include any changes to libraries upon which we depend. We should make separate tasks for each library which has warnings to update them to the latest version. If the warnings still exist, we should try to get them fixed upstream.

Next steps

This is going to be a long slog, but it turns out, the QT Creator IDE actually does make it a little easier to jump to the correct file and fix things more quickly.