See README-CardDAV.md for details of the implementation.

New files:
- libbalsa-webdav.[hc]: WebDAV base implementation
- libbalsa-carddav.[hc]: CardDAV implementation
- address-book-carddav.[hc]: Balsa CardDAV addressbook implementation
- documentation of CardDAV support

Changed files:
- configure.ac, meson.build, meson_options.txt, libbalsa/Makefile.am,
  libbalsa/meson.build: add new module to configuration and build
- README.md: update general build and usage documentation
- libbalsa/address.c: add comment re. VCARD 4.0 (RFC 6350)
- libbalsa/libbalsa.[hc]: initialise CardDAV support if configured
- libbalsa/rfc6350.[hc]: add method to create a VCARD 3 or 4 from a
  LibBalsaAddress
- src/address-book-config: add CardDAV address book config GUI
- src/main.c: initialise libxml if CardDAV is enabled
- src/pref-manager.c: add CardDAV address book