Make PlasmaActivities more optional [WIP]

[a17r]

• [x] kde-plasma/libplasma (see also: https://invent.kde.org/plasma/libplasma/-/merge_requests/1364)
• [x] kde-plasma/kwin and kde-plasma/kwin-x11
• [x] kde-plasma/powerdevil (needs small-ish patch)
• [ ] kde-plasma/plasma-browser-integration (needs patching)
• [ ] kde-plasma/plasma-desktop (huge): imports/activitymanager, desktoppackage/contents/activitymanager, kcms/activities; patching in many other places
• [ ] kde-plasma/plasma-workspace (huge): containmentactions/switchactivity; patching in many other places, however there is https://invent.kde.org/plasma/plasma-workspace/-/issues/35
• [x] kde-apps/okular ~~(see also: https://invent.kde.org/graphics/okular/-/merge_requests/1233)~~ dependency dropped upstream in https://invent.kde.org/graphics/okular/-/merge_requests/1234