gdb: More cleanup changes

- Move the message handler into its debugger class part,
- Move all message types into one file and collapse 3 of the ones with no data into a generic, stateless message with a single property being its type,
- Add an Fpscr helper property on IExecutionContext along with a comment about what Fpscr is (similar to the other registers in there)
- Moved the Rcmd helpers (such as GetRegisters, GetMinidump, etc) into a dedicated Debugger class part,
- Fixed the double-collection (ToArray being called twice) in GetThreadUids & GetThread in KProcess

src/Ryujinx.HLE/Debugger/IDebuggableProcess.cs

@@ -11,11 +11,11 @@ namespace Ryujinx.HLE.Debugger
         void DebugContinue(KThread thread);
         bool DebugStep(KThread thread);
         KThread GetThread(ulong threadUid);
-        DebugState GetDebugState();
         bool IsThreadPaused(KThread thread);
-        ulong[] GetThreadUids();
         public void DebugInterruptHandler(IExecutionContext ctx);
         IVirtualMemoryManager CpuMemory { get; }
+        ulong[] ThreadUids { get; }
+        DebugState DebugState { get; }
         void InvalidateCacheRegion(ulong address, ulong size);
     }
 }