Add Xorg modifiers (#57)

* implement additional modifiers to xorg.

* xorg modifiers to API.

* update modifiers api & add ws.

* scroll pos rename to delta and add ctrl key.

internal/webrtc/payload/receive.go

@@ -21,11 +21,18 @@ type Move struct {
 	Y uint16
 }
 
-type Scroll struct {
+// TODO: remove this once the client is fixed
+type Scroll_Old struct {
 	X int16
 	Y int16
 }
 
+type Scroll struct {
+	DeltaX     int16
+	DeltaY     int16
+	ControlKey bool
+}
+
 type Key struct {
 	Key uint32
 }