fix(activatable): add listeners for custom activators

[johnleider]

Description

Add listeners to custom activators

fixes #7944
fixes #5890
fixes #5860

Motivation and Context

Custom activators were not working at all

How Has This Been Tested?

jest

Markup:

<template>
  <div class="ma-5 pa-5">
    <v-container>
      <v-btn
        id="activator-click"
        ref="act"
        color="success"
      >
        click
      </v-btn>

      <v-menu
        :activator="activator"
      >
        <v-card>
          <v-card-text>
          </v-card-text>
        </v-card>
      </v-menu>
      <div class="ma-5"></div>
      <v-btn
        id="activator-hover"
        color="error"
      >
        hover
      </v-btn>
      <v-menu
        activator="#activator-hover"
        open-on-hover
      >
        <v-card>
          <v-card-text>
          </v-card-text>
        </v-card>
      </v-menu>
    </v-container>
  </div>
</template>

<script>
  export default {
    data: () => ({
      activator: null,
    }),

    mounted () {
      this.activator = this.$refs.act
    },
  }
</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)

[codecov]

Codecov Report

Merging #8121 into master will decrease coverage by 0.21%.
The diff coverage is 90.9%.

@@            Coverage Diff             @@
##           master    #8121      +/-   ##
==========================================
- Coverage   85.76%   85.54%   -0.22%     
==========================================
  Files         334      334              
  Lines        9096     9388     +292     
  Branches     2420     2535     +115     
==========================================
+ Hits         7801     8031     +230     
- Misses       1207     1268      +61     
- Partials       88       89       +1

Impacted Files	Coverage Δ
packages/vuetify/src/mixins/menuable/index.ts	91.66% <ø> (ø)	⬆️
packages/vuetify/src/components/VMenu/VMenu.ts	71.51% <100%> (+0.18%)	⬆️
packages/vuetify/src/mixins/activatable/index.ts	95.58% <90.62%> (-4.42%)	⬇️
...uetify/src/components/VCalendar/VCalendarWeekly.ts	72.72% <0%> (-6.59%)	⬇️
packages/vuetify/src/mixins/dependent/index.ts	100% <0%> (ø)	⬆️
packages/vuetify/src/services/theme/index.ts	100% <0%> (ø)	⬆️
...ges/vuetify/src/components/VCalendar/util/props.ts	100% <0%> (ø)	⬆️
packages/vuetify/src/components/VSelect/VSelect.ts	94.13% <0%> (+0.36%)	⬆️
packages/vuetify/src/components/VWindow/VWindow.ts	98.19% <0%> (+0.76%)	⬆️
packages/vuetify/src/components/VApp/VApp.ts	90% <0%> (+1.11%)	⬆️
... and 4 more

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