debugger: Rework debugger for improved stability

This is a first pass for now, more work is needed mostly for UI polish.
Changes in this commit:
- Debugger now exposes a thread-safe API. Previously everything would just access internal debugger state directly without regard for race conditions
- Reworked stepping logic to be more reliable. For example previously Cemu could get forever stuck in the middle of stepping to the next instruction
- Support for debugging while using multi-threaded CPU emulation. Originally the debugger was added when only single core support existed in Cemu. It was possible to debug multi-threaded before but it was very very prone to state corruptions.
- Debugger should now remember breakpoints correctly across sessions