Make PlasmaActivities more optional [WIP]

[a17r]

• kde-plasma/libplasma (see also: https://invent.kde.org/plasma/libplasma/-/merge_requests/1364)
• kde-plasma/kwin and kde-plasma/kwin-x11
• 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
• 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

[Chiitoo]

So far tested:

• kde-plasma/libplasma-6.4.5-r1
• kde-plasma/kwin-x11-6.4.5-r1
• kde-apps/okular (25.08.1 not available so used 25.08.0 instead)
• kde-plasma/systemsettings-6.4.5 (just quickly hacked activities out)

These were the only things pulling in activities for me (now dep-cleaned).

While testing, I found a segfault lurking between 'Animations' and possibly any other setting category, just switching between them... unsure if it could be related to the hacking (bunches of Qml in the backtrace... will look more at another time).

Thank you!