fix(VSlideGroup): recalculate widths when it shows

[beicause]

Description

fixes #14195

Motivation and Context

How Has This Been Tested?

Markup:

<template>
  <v-container>
    <v-btn @click="show = !show">
      Show bug
    </v-btn>
    <v-sheet
      elevation="5"
      class="ma-5 pa-5"
      max-width="800"
    >

      <v-slide-group
        v-show="show"
        show-arrows
      >
        <v-slide-item
          v-for="n in list"
          :key="n"
          v-slot="{ active, toggle }"
        >
          <v-btn
            class="mx-2"
            :input-value="active"
            active-class="primary white--text"
            depressed
            rounded
            @click="toggle"
          >
            Options {{ n }}
          </v-btn>
        </v-slide-item>
      </v-slide-group>
    </v-sheet>
  </v-container>
</template>

<script>
  export default {
    data: () => ({
      show: false,
      list: Array.from(Array(10).keys()),
    }),
  }
</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 backwards compatible changes and next for non-backwards compatible 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)

[Goshin]

Would this solution still work if we toggle the parent's display property instead of that of the VSlideGroup itself?

[beicause]

Would this solution still work if we toggle the parent's display property instead of that of the VSlideGroup itself?

it's really a problem😅