Show nested folders in App List

[Zren]

I'd need to refactor AppsModel.qml to parse a X-deep tree instead of a 2-depth tree.

• https://github.com/KDE/plasma-workspace/blob/master/applets/kicker/plugin/rootmodel.h
• https://github.com/KDE/plasma-workspace/blob/master/applets/kicker/plugin/appsmodel.h
• https://github.com/KDE/plasma-workspace/blob/master/applets/kicker/plugin/appsmodel.cpp

[Zren]

I once wrote a recursive iterApp() function to display the AppList like so in a mockup project in 2017/2018 called tiledmenu3.

• https://gist.github.com/Zren/8b84ffbb540e7e1bda788dcfca11c074