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/HOS/Kernel/Threading/KScheduler.cs

@@ -293,7 +293,7 @@ namespace Ryujinx.HLE.HOS.Kernel.Threading
             KThread currentThread = KernelStatic.GetCurrentThread();
             KThread selectedThread = _state.SelectedThread;
 
-            if (!currentThread.IsThreadNamed && currentThread.GetThreadName() != "")
+            if (!currentThread.IsThreadNamed && string.IsNullOrEmpty(currentThread.GetThreadName()))
             {
                 currentThread.HostThread.Name = $"<{currentThread.GetThreadName()}>";
                 currentThread.IsThreadNamed = true;