Merge Branch 'homescreen-ng'

This replaces the home screen to solve KTouch's most egrigous problem:
content discovery. Previously, KTouch displayed only the courses for the
the currently active keyboard layout. If KTouch has no course for this
layout, the windows was basically empty showing only the option to
create a new custom lesson. Being forced to set up first the correct
keyboad layout for your computer has proven to be unintuitive. The
new home screen shows all available training content at a glance.

Improvements in more detail:

 * There is a new sidebar allowing the user to select from all available
   keyboard layouts with their associated courses. Users still have to
   set up the correct layout matching the selected course, though, and
   they are told how to do so with an passive notification. (BUG 395264,
   BUG 385425, BUG 331368, BUG 398773, BUG 391968, BUG 364011)

 * Training statistics are now available from the home screen for each
   lesson. Previously they have been only shown on the score screen
   after completing a lesson. (BUG 401359, BUG 317174)

 * The lesson selector has been redesign to show previews of each lesson
   as selectable items in a grid view.

  * Tab focus works now as expected and allows training without using the
    mouse at all. (BUG 316058)

  * Support for HiDPI rendering has been added.

  * KTouch now uses Qt Quick Controls 2 instead of version 1.

BUG: 385425
BUG: 401359
BUG: 331368
BUG: 398773
BUG: 391968
BUG: 364011
BUG: 316058
BUG: 395264
BUG: 317174
FIXED-IN: 19.08.0
Differential Revision: https://phabricator.kde.org/D20953