Add wheel event supportfor htmlbackend

Author: alt-tosc

src/fidget/dom2.nim

@@ -771,6 +771,8 @@ type
     target*: Node
     `type`*: cstring
     isTrusted*: bool
+    deltaX*: float32
+    deltaY*: float32
 
   UIEvent* = ref UIEventObj ## see `docs<https://developer.mozilla.org/en-US/docs/Web/API/UIEvent>`_
   UIEventObj {.importc.} = object of Event

src/fidget/htmlbackend.nim

@@ -519,6 +519,11 @@ proc startFidget*(draw: proc(), load: proc() = nil, w = 0, h = 0) =
     ## Scroll does not need to do anything special in HTML mode
     refresh()
 
+  dom.window.addEventListener "wheel", proc(event: Event) =
+    ## When wheel is used
+    mouse.wheelDelta += vec2(event.deltaX, event.deltaY)
+    refresh()
+
   dom.window.addEventListener "mousedown", proc(event: Event) =
     ## When mouse button is pressed
     let event = cast[MouseEvent](event)