For thread safety, we shouldn't return any pointers or references that
can be used to mutate the state of the PPCSymbolDB. This should be the
final part of making PPCSymbolDB thread safe unless I've missed
something.

9395238 added locking in some PPCSymbolDB functions that access member
variables, but far from all. To ensure thread safety, this commit adds
the missing locking.

By building the map in a local variable and then swapping it with the
member variable, we avoid the need to hold a lock while building the
map.

Cleanup loading code and reduce amount of signals.

On boot. allow previously loaded map to be kept, if its filename matches. Useful for restarting a game with a large symbol map.

Notes are separate from function symbols, and can be searched separately.
Unlike functions, notes of different length can overlap each other.
In the instruction window, a note will always display over the function symbol.

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

Continues the migration of our code over to the fmt logger.

These strings are only used for comparison against other strings, so a
string view can be used here.

Otherwise DolphinQt will have a stale symbol list and
you can get nullptr dereferences when trying to use it.