nost: touch piano mode (#830)

## Link to GitHub Issue or related Pull Request, if one exists
n/a

## Description of change
Touch piano mode. Allows piano keys to be played with the touch screen.
Since the touch screen is still needed for menu navigation (including
during songs) a toggle switch is added so that the user can flip between
navigation mode and piano mode.

The touch targets are pixel-perfect. Don't let the game's piano
animations fool you.

## Testing

src/spice2x/overlay/overlay.h

@@ -83,6 +83,7 @@ namespace overlay {
         // not part of `windows`: drawn/updated on the persistent layer (like
         // notifications), independent of the overlay's active state and input gates.
         std::unique_ptr<Window> window_fps;
+        std::unique_ptr<Window> window_nostalgia_touch_piano;
 
         // not part of `windows`: the main menu / launcher. owned and drawn
         // separately from the overlay window layer; it drives the overlay's
@@ -138,6 +139,9 @@ namespace overlay {
         inline IDirect3DDevice9 *get_device() {
             return this->device;
         }
+        inline HWND get_window() {
+            return this->hWnd;
+        }
 
         bool can_transform_touch_input() {
             return (this->subscreen_mouse_handler != nullptr);