Fix a crash when closing the main UI (#904)

* Fix a crash when closing the main Ui

Also make sure to dispose the OpenAL context to not leak memory when
unloading the emulation context.

* Improve keys and 'game already running' dialogs

* Make sure to dispose the page table and ThreadContext

Less memory leaks!

* Fix tests

* Address gdk's comments

Ryujinx.HLE/HOS/Kernel/Process/KProcess.cs

@@ -1131,5 +1131,10 @@ namespace Ryujinx.HLE.HOS.Kernel.Process
         {
             throw new UndefinedInstructionException(e.Address, e.OpCode);
         }
+
+        protected override void Destroy()
+        {
+            CpuMemory.Dispose();
+        }
     }
 }