third attempt at Cards rework

This MR breaks how Card resizes because it will have a fixed size set by default (or by the user), so I suggest this MR to be merged in KF6.

• Calculate card width in CardsGridView to reduce calculations made per size change, use card size as a base to calculate cell size
• New widget: CardTemplate. This widget includes a background (and exposing its properties to the user) and includes some generic size setters.
• BannerImage: allow positioning the title at vertical center, if title background does not cover enough space, it can expand horizontally to cover all of it.
• AbstractCard:
  • better visible setters for parent items
  • strict column and row positioning
  • parent item size constraints: maximum, minimum parent item size properties
  • update doc

How to expose these chanegs to the users ?

• kirigami-gallery
  • Needs to set CardsGridViewGallery's Label in a Card to elide to the right
  • CardsGridView gallery needs to show that changing card size is indeed possible (maximum, minimum size, or even static size)
  • CardsLayoutGallery needs to showcase an example of CardTemplate
  • Showcase changes to BannerImage with options to customize BannerImage title (color, word wrap, elide), title background (color, opacity, horizontal expansion)
• kirigami documentation
  • CardTemplate documentation doesn't exist
  • CardsGridView documentation needs an update (card size)