QmlWeb.Dom/DomElement: support setting attrs and style

Example:

```
import QtQuick 2.0
import QmlWeb.Dom 1.0

DomElement {
  width: 100
  height: 200
  attrs.id: 'myElement'
  style.background: 'green'
 }
```

This works only on DomElement elements (from QmlWeb.Dom) module which
contains DOM-specific components and is not interoperable with Qt QML.

It is, however, possible to at least partially mock QmlWeb.Dom behavior
with QtQuick QML, but that's not in scope.

src/modules/QmlWeb.Dom/DomElement.js

@@ -1,13 +1,22 @@
 // eslint-disable-next-line no-undef
 class QmlWeb_Dom_DomElement extends QtQuick_Item {
   static properties = {
+    attrs: { type: "var", initialValue: {} },
+    style: { type: "var", initialValue: {} },
     tagName: { type: "string", initialValue: "div" }
   };
 
   constructor(meta) {
     meta.tagName = meta.object.tagName;
     super(meta);
 
-    // TODO: support properties, styles, perhaps changing the tagName
+    for (const key in meta.object.attrs) {
+      if (!meta.object.attrs.hasOwnProperty(key)) continue;
+      this.dom[key] = meta.object.attrs[key];
+    }
+    for (const key in meta.object.style) {
+      if (!meta.object.style.hasOwnProperty(key)) continue;
+      this.dom.style[key] = meta.object.style[key];
+    }
   }
 }