Update appdata to use keywords and include Kalendar in description

Kalendar was included as part of the Kontact flatpak bundle in May 2022 (https://invent.kde.org/pim/kalendar/-/issues/68). Currently searching "Kalendar" doesn't return this bundle either within Discover or Flathub's search. This bundle issue is addressed by using the keyword spec (https://www.freedesktop.org/software/appstream/docs/chap-CollectionData.html#tag-ct-keywords) and adding Kalendar to the description