Add hover highlight and link dragging features (#222)

* Add new hover highlight and link dragging features

- Hovering effects added for node inputs and outputs
- Adds "mouseout" handler to properly clear node state when mousing off a node
- Fixes bug that causes outputs to always be dimmed out when connecting from inputs
- Slight performance improvement

* Add node highlight text colour option

* Add slot highlight colour fallback

src/LGraphNode.ts

@@ -144,7 +144,7 @@ export class LGraphNode {
     _shape?: RenderShape
     subgraph?: LGraph
     skip_subgraph_button?: boolean
-    mouseOver?: boolean
+    mouseOver?: IMouseOverData
     is_selected?: boolean
     redraw_on_mouse?: boolean
     // Appears unused