[Applet] Use the more user-friendly term "recording" instead of "capture"

Summary:
"Capture Devices" sounds like some kind of kidnapping tool to people who aren't
audio experts. Changing "Capture" to "Recording" both makes the text less scary,
and also improves clarity ("Capture" only implies that it's for audio capture;
"Recording" makes it explicit)

Reviewers: Plasma, VDG, drosca, filipf, davidedmundson

Reviewed By: VDG, filipf

Subscribers: davidedmundson, filipf, broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D22453