Port Language KCM to Qt Quick

Summary:
* Changed the overall design from two lists to one list with a modal
  sheet to add more languages.
* Replaced a modal "You need to relogin for changes" dialog with a
  MessageType.Positive InlineMessage.
* Reworked the way missing languages are handled: The old KCM silently
  rewrote config and showed a warning. The new design shows an
  informative warning and removes the missing languages on the next
  save. Until then they're flagged as missing in the list.
* Manages Apply button state correctly (or rather at all ...).

This depends on D12097.

This implements T7247.

This is currently not final code. It's a WIP upload to give Marco
something to work with to fix various Kirigami and SimpleKCM problems.

Currently known issues:
* Does not save (code is from old KCM, might have been broken there)
* Disabled SwipeListItem actions do not show disabled
* Placement of actions button in SwipeListItem is wonky if the
  contentItem is a RowLayout
* SwipeListItem spews errors about positionAnimation after using an
  action
* SwipeListItem is awkward to use, we need a drag-reorderable list
  delegate
* OverlaySheet spews numerous warnings about not being able to find
  applicationWindow and activeFocusItem
* The sheet is parented to the SimpleKCM's parent since there's no
  app window to be modal too
* The footer inside an OverlaySheet sometimes moves up above the
  content instead of staying down
* Even though SimpleKCM is just a Kirigami.ScrollablePage like
  Kirigami Gallery pages, an InlineMessage that fills the page width
  gets cut off on the left and right, so wonky code to insert margins
  next to them
* List has window bg color as background instead of view background
  color

Reviewers: #kirigami, mart

Subscribers: plasma-devel

Tags: #plasma, #kirigami

Differential Revision: https://phabricator.kde.org/D12102