rewinder UI - limit target frame length to 500000

I guess the 16383 came from looking at the code, but 16383 is a limit on the number of states (which also needs to be increased, TODO) not how many frames back those states go.  There is effectively no limit except numerical calculation limits on the target frame length; if you had room for only 10 savestates, but wanted to cover a 10 million frame range, we'd just save once every million frames

src/BizHawk.Client.EmuHawk/config/RewindConfig.Designer.cs

@@ -211,7 +211,7 @@
 			// 
 			this.TargetFrameLengthNumeric.Location = new System.Drawing.Point(125, 135);
 			this.TargetFrameLengthNumeric.Maximum = new decimal(new int[] {
-            16383,
+            500000,
             0,
             0,
             0});