Rewrite Window List plasmoid.

• Ported to libtaskmanager-ng, dropping dependency on legacytaskmanager
• Made virtual desktop sectioning in the listview actually work
  • Bonus: Don't section when there is only one virtual desktop
• Made Unclutter/Cascade window actions actually work, and fix the wonky way they were shown in the UI
  • Code added to KWindowSystem QML plugin
• Implemented full keyboard navigation: enter/return, arrows and tab/backtab
• Added popup window pin
• Added basic Accessibility support
• Made UI sizes and margins consistent with Kicker and Folder View listviews

Differential Revision: https://phabricator.kde.org/D2089