Add custom refresh rate mode to VSync option (#238)

Rebased @jcm93's refreshinterval branch:
https://github.com/jcm93/Ryujinx/tree/refreshinterval

The option is placed under System/Hacks. Disabled, it's the default
Ryujinx behavior. Enabled, the behavior is shown in the attached
screenshots. If a framerate is too high or low, you can adjust the value
where you normally toggle VSync on and off. It will also cycle through
the default on/off toggles.

Also, in order to reduce clutter, I made an adjustment to remove the
target FPS and only show the percentage.

---------

Co-authored-by: jcm <6864788+jcm93@users.noreply.github.com>

src/Ryujinx/Common/KeyboardHotkeyState.cs

@@ -3,7 +3,7 @@ namespace Ryujinx.Ava.Common
     public enum KeyboardHotkeyState
     {
         None,
-        ToggleVSync,
+        ToggleVSyncMode,
         Screenshot,
         ShowUI,
         Pause,
@@ -12,5 +12,7 @@ namespace Ryujinx.Ava.Common
         ResScaleDown,
         VolumeUp,
         VolumeDown,
+        CustomVSyncIntervalIncrement,
+        CustomVSyncIntervalDecrement,
     }
 }