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/Threading/KThread.cs

@@ -1141,6 +1141,8 @@ namespace Ryujinx.HLE.HOS.Kernel.Threading
         {
             Owner.Translator.Execute(Context, entrypoint);
 
+            Context.Dispose();
+
             ThreadExit();
         }