In plasma code, it's more common to see `/** ... */` doxygen comments
rather than `/** ... **/`.
This change fixes up doxygen comment terminators to prevent inconsistency
issues.
- 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>