v1.2.0

Fixes from running the examples against the real API.

Added

• MaxBot.deleteWebhook() (and deleteWebhookBlocking()) removes every webhook subscription of
  the bot.

Fixed

• MessageContext.text is null for photos, stickers and other messages without text. Max sends an
  empty string for them, so text ?: "..." replied with an empty message (rejected with
  400 proto.payload) and Filters.hasText() let such messages through.
• Long polling no longer receives nothing while the bot is subscribed to a webhook: Max answers
  getUpdates with empty pages then, so startPolling() now checks the subscriptions first and
  throws MaxException, as documented.
• sendMessage rejects a message without text, attachments or a forwarded message with
  IllegalArgumentException instead of sending it and getting 400 proto.payload: errors.required.
• The Ktor example unsubscribes from the webhook when it stops and logs through slf4j-simple.
• The echo examples change the message on "Like" too, since the web client of Max does not show
  callback notifications.

Changed

• Documented that the web client of Max shows neither the command menu nor callback notifications.