Migrate zwp_text_input_v2 to new approach

- Drop the v0 support, it is no longer used by anything
- Adapt the text-input related methods in seat interface to include
  versioning, this will be useful for when zwp_text_input_v3 support is
  included in kwayland-server
- Refactor SeatInterface to get rid of globalTextInput, it is no longer
  needed with the new approach
- Refactor out the ContentHints and ContentPurpose enums in separate
  header file which can be shared by zwp_text_input_v2/3 implementation

Co-Author: Bhushan Shah <bshah@kde.org>