Fix inconsistencies in progress reporting (#2129)

Signal and setup events correctly
Eliminate possible races
Use a single event
Mark volatiles and reduce scope of waithandles
Common handler
100ms -> 50ms

Ryujinx.Graphics.Gpu/Shader/ShaderCacheState.cs

@@ -0,0 +1,13 @@
+namespace Ryujinx.Graphics.Gpu.Shader
+{
+    /// <summary>Shader cache loading states</summary>
+    public enum ShaderCacheState
+    {
+        /// <summary>Shader cache started loading</summary>
+        Start,
+        /// <summary>Shader cache is loading</summary>
+        Loading,
+        /// <summary>Shader cache finished loading</summary>
+        Loaded
+    }
+}