Replace global event buses with mitt

[st3iny]

Ref https://v3-migration.vuejs.org/breaking-changes/events-api.html#event-bus Ref https://github.com/developit/mitt

TODO

• [x] Calendar (no global event bus)
• [x] Contacts: https://github.com/nextcloud/contacts/pull/3766
  • contacts/src/components/ContactDetails/ContactDetailsAddNewProp.vue
  • contacts/src/components/ContactDetails/ContactDetailsProperty.vue
  • contacts/src/components/ContactsList/ContactsListItem.vue
  • contacts/src/components/Properties/PropertyTitle.vue
• [x] Mail: https://github.com/nextcloud/mail/pull/9211
  • mail/src/components/Composer.vue
  • mail/src/components/ComposerAttachment.vue
  • mail/src/components/ComposerAttachments.vue
  • mail/src/components/EnvelopeList.vue
  • mail/src/components/Mailbox.vue
  • mail/src/components/MailboxThread.vue
  • mail/src/components/NavigationMailbox.vue
  • mail/src/components/OutOfOfficeForm.vue
  • mail/src/components/TextEditor.vue
  • mail/src/directives/drag-and-drop/draggable-envelope/draggable-envelope.js
  • mail/src/directives/drag-and-drop/droppable-mailbox/droppable-mailbox.js
  • mail/src/directives/drag-and-drop/util/dragEventBus.js
  • mail/src/directives/infinite-scroll.js
  • mail/src/tests/unit/components/TextEditor.spec.js