feat: Added the ability to use custom button icons

[ab3lson]

Summary

This PR adds the ability to customize button icons and styles, allowing users to repurpose media control buttons and other buttons for different functions (e.g., using PLAY/PAUSE/STOP for receiver volume control).

Changes

• Editor UI: Added a new collapsible "Overrides" section in the visual editor for configuring button overrides without editing YAML
• Icon customization: Users can set custom Material Design icons for any button via the icon property
• Style customization: Users can apply custom CSS styles to buttons via the style property
• Documentation: Updated Override_buttons.md with examples and a configuration options table

Example Usage

keys:
  PLAY:
    icon: mdi:volume-high
    service: media_player.volume_up
    data:
      entity_id: media_player.receiver
  PAUSE:
    icon: mdi:volume-mute
    service: media_player.volume_mute