QTree expand/collapse bug and how to fix it

[MTLink]

QTree expand/collapse functionality doesn't work in some cases. For example if tree model is dynamicaly created. The reason for this is an error in quasar/src/components/tree/TreeItem.vue
Vue framework doesn't allow to change props inside component
this.model.expanded = !this.model.expanded -- not correct

Therefore it is needed add

data: function () {
  return { expanded: this.model.expanded }
}

And then use expanded instead of model.expanded

[rstoenescu]

Available in future v0.15