Tree model-value does not track multiple selections

[stevage]

You can see this in the demo page: https://antoniandre.github.io/wave-ui/w-tree#v-model

Select two items. They are visually both selected. But the "Selection" panel (ie, the value of v-model) only shows the value of the most recently clicked item.

So this component is not reporting the current state of which items are selected.

A few other issues with the tree component while I'm here:

• there's no way to disable multi-select
• there's no way to change the behaviour that clicking a parent both expands it and selects it. You can't prevent the parent being selected.
• there's no way to programmatically select/unselect an item (to work around some of the above)

Love the library though, I use it a lot.