fix(VTreeview): don't emit events on mount

[nekosaur]

Description

fix treeview emitting events on mount

Motivation and Context

solves #7296

How Has This Been Tested?

playground

Markup:

<template>
  <v-container>
    <v-treeview
         :items="items"
         :active="selected"
         v-model="selected"
         selectable
         @input="logInput($event)"
        >
        </v-treeview>
  </v-container>
</template>

<script>
  export default {
    data: () => ({
      selected: [6],
      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' }
            ]

          }
        ]
    }),
    beforeMount:function(){
      console.log('Component beforeMount');
    },
    mounted:function(){
      console.log('Component Mounted');
    },
    methods:{
      logInput(e){
        console.log('Input Event Fired');
      }
    }
  }
</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 feature but make 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)