fix: generalize fix for swallowed hover events

Replaces the old drag hover fix (which fixed kindof the same thing)
with one that works in a general fashion only on element.* level.

Closes bpmn-io/bpmn-js#1366

----

MISSING:

* tests

lib/features/dragging/HoverFix.js

@@ -79,59 +79,48 @@ export default function HoverFix(eventBus, dragging, elementRegistry) {
 
 
   /**
-   * We make sure that drag.out is always fired, even if the
+   * We make sure that element.out is always fired, even if the
    * browser swallows an element.out event.
    *
    * Event sequence:
    *
-   * drag.hover
+   * element.hover
    * (element.out >> sometimes swallowed)
-   * element.hover >> ensure we fired drag.out
+   * element.hover >> ensure we fired element.out
    */
-  eventBus.on('drag.init', function() {
+  (function() {
+    var hoverGfx;
+    var hover;
 
-    var hover, hoverGfx;
+    eventBus.on('element.hover', function(event) {
 
-    function setDragHover(event) {
-      hover = event.hover;
-      hoverGfx = event.hoverGfx;
-    }
+      // (1) remember current hover element
+      hoverGfx = event.gfx;
+      hover = event.element;
+    });
 
-    function unsetHover() {
-      hover = null;
-      hoverGfx = null;
-    }
+    eventBus.on('element.hover', HIGH_PRIORITY, function(event) {
 
-    function ensureOut() {
+      // (3) am I on an element still?
+      if (hover) {
 
-      if (!hover) {
-        return;
+        // (4) that is a problem, gotta "simulate the out"
+        eventBus.fire('element.out', {
+          element: hover,
+          gfx: hoverGfx
+        });
       }
 
-      var element = hover,
-          gfx = hoverGfx;
-
-      hover = null;
-      hoverGfx = null;
-
-      // emit synthetic out event
-      dragging.out({
-        element: element,
-        gfx: gfx
-      });
-    }
+    });
 
-    eventBus.on('drag.hover', setDragHover);
-    eventBus.on('element.out', unsetHover);
-    eventBus.on('element.hover', HIGH_PRIORITY, ensureOut);
+    eventBus.on('element.out', function() {
 
-    eventBus.once('drag.cleanup', function() {
-      eventBus.off('drag.hover', setDragHover);
-      eventBus.off('element.out', unsetHover);
-      eventBus.off('element.hover', ensureOut);
+      // (2) unset hover state if we correctly outed us *GG*
+      hoverGfx = null;
+      hover = null;
     });
 
-  });
+  })();
 
   this._findTargetGfx = function(event) {
     var position,