feat(VTextField): only show placeholder on focus if label is set

[KaelWD]

Motivation and Context

resolves #12499

Having both label and placeholder displayed can be a usability problem as the placeholder looks very similar to typed text and could be confused with user input at a glance.

Potentially breaking change, you can't show both label and placeholder any more but this should be discouraged anyway.

EDIT: This is actually MD2 spec now:

Markup:

<template>
  <v-container>
    <v-text-field label="Label"></v-text-field>
    <v-text-field
      label="Label"
      placeholder="Placeholder"
    ></v-text-field>
    <v-text-field
      label="Label"
      placeholder="Placeholder"
      single-line
    ></v-text-field>
    <v-text-field
      label="Label"
      placeholder="Placeholder"
      value="Value"
    ></v-text-field>
  </v-container>
</template>

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)