Port ExternalAppModule to QML
As part of the transition to QML KCMs over Qt Widgets, this would need to be replaced with QML. Ideally, I would imagine this using a PlaceholderMessage.
It seems the only uses are System Monitor in KInfoCenter (and YaST in System Settings? kcmutils/autotests/YaST-systemsettings.desktop
).
An alternative approach to creating a QML UI might be to remove this feature entirely and create shallow KCMs for when we want to link to an external application. It could even be a Kirigami KCM type. It seems like we only really want to do this for System Monitor, and maybe we don't want links to non-Plasma/KDE applications at all. If the end goal is to replace System Settings with Plasma Mobile's plasma-settings, then this might be easiest.