A new debugger.

Lots of bugs/rough edges/etc - issues will be filed.
Old-style debugging still works (just use --emit_source_annotations to get
the helpful movs back and --break_on_instruction will still fire).

src/xenia/ui/loop_win.h

@@ -26,6 +26,8 @@ class Win32Loop : public Loop {
   Win32Loop();
   ~Win32Loop() override;
 
+  bool is_on_loop_thread() override;
+
   void Post(std::function<void()> fn) override;
   void PostDelayed(std::function<void()> fn, uint64_t delay_millis) override;