fix(VTreeview): indeterminate state no longer based on parent

[skycow]

Description

A Treeview node no longer sets it's indeterminate state based on it's parent. The indeterminate
state is only set by the default state, the node's previous state, or by the states of the node's
children. A new leaf node with no children that is added will now only be set to selected if the parent is selected and will not be selected if the parent is not selected or indeterminate. If a new node does have children, it can only get set to indeterminate based on the children, not on the parent.

Motivation and Context

fix #8256

How Has This Been Tested?

Visually

Markup:

<template>
  <v-container>
    <v-treeview
      selectable
      :items="items"
    ></v-treeview>
    <v-btn
      width="200"
      color="primary"
      @click="addItem"
    >Add Item</v-btn>
  </v-container>
</template>

<script>
export default {
  data: () => ({
    maxId: 25,
    items: [
      {
        id: 1,
        name: 'Applications :',
        children: [
          { id: 2, name: 'Calendar : app' },
          { id: 3, name: 'Chrome : app' },
          { id: 4, name: 'Webstorm : app' },
        ],
      },
      {
        id: 5,
        name: 'Documents :',
        children: [
          {
            id: 6,
            name: 'vuetify :',
            children: [
              {
                id: 7,
                name: 'src :',
                children: [
                  { id: 8, name: 'index : ts' },
                  { id: 9, name: 'bootstrap : ts' },
                ],
              },
            ],
          },
          {
            id: 10,
            name: 'material2 :',
            children: [
              {
                id: 11,
                name: 'src :',
                children: [
                  { id: 12, name: 'v-btn : ts' },
                  { id: 13, name: 'v-card : ts' },
                  { id: 14, name: 'v-window : ts' },
                ],
              },
            ],
          },
        ],
      },
      {
        id: 15,
        name: 'Downloads :',
        children: [
          { id: 16, name: 'October : pdf' },
          { id: 17, name: 'November : pdf' },
          { id: 18, name: 'Tutorial : html' },
        ],
      },
      {
        id: 19,
        name: 'Videos :',
        children: [
          {
            id: 20,
            name: 'Tutorials :',
            children: [
              { id: 21, name: 'Basic layouts : mp4' },
              { id: 22, name: 'Advanced techniques : mp4' },
              { id: 23, name: 'All about app : dir' },
            ],
          },
          { id: 24, name: 'Intro : mov' },
          { id: 25, name: 'Conference introduction : avi' },
        ],
      },
    ],
  }),
  methods: {
    addItem() {
      this.maxId += 1;
      this.items[0].children.push({ id: this.maxId, name: "Another: app" });
    }
  }
}
</script>

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any features but makes things better)

Checklist:

• The PR title is no longer than 64 characters.
• The PR is submitted to the correct branch (master for bug fixes and documentation updates, dev for new features and breaking changes).
• My code follows the code style of this project.
• I've added relevant changes to the documentation (applies to new features and breaking changes in core library)
• I've added new examples to the kitchen (applies to new features and breaking changes in core library)

[jacekkarczmarczyk]

Can you add a unit test?

[johnleider]

@skycow please add a corresponding unit test

[johnleider]

Fixed in adbb374

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