Make important desktop functionality not depend on the presence of applets

A lot of what we consider baseline features in a modern desktop — volume keys, media keys, clipboard management, notifications — are handled by the applets in Plasma.

This implementation detail creates a very common pathway for users to break their systems by removing applets because they don't use the applets themselves, not knowing that those applets are the sole providers of related functionality in Plasma.

Proposal: we move appropriate parts of the applets into KDED services.

Applets which need this treatment:

• Media Player applet provides media keys (bug 409190, bug 460771)
• Audio Volume applet provides volume keys (bug 391578, plasma-pa!221 (merged))
• Clipboard applet provides clipboard management (bug 443036)
• Notifications applet provides notifications (this might be difficult)

Former metabug about this on Bugzilla: https://bugs.kde.org/show_bug.cgi?id=443036