hid: Initial Keyboard Support (#684)

* hid: Initial Keyboard Support

This adds basic hid keyboard support.

Because of OpenTK.Input limitations, some specials keys aren't mapped.

* Fix code style

* Fix for loops code style

* Make hid keyboard feature toggleable

* Address comments

* Fix 2 other nits

* Apply jd's suggestion

Ryujinx.HLE/Input/HidKeyboard.cs

@@ -0,0 +1,8 @@
+namespace Ryujinx.HLE.Input
+{
+    public struct HidKeyboard
+    {
+        public int   Modifier;
+        public int[] Keys;
+    }
+}