Add support for KUserFeedback
This commit introduces KUserFeedback in dolphin with some basic data sources and with a settings page to configure the telemetry values. There are also a couple custom data sources as proof of concept: a bunch of settings and the count of available network shares as listed by Solid. The settings page is shown only if the user feedback framework is enabled, but currently in Plasma we don't have a global kill switch to disable it. At the moment we never show an encouragement message. We need to connect to the `Provider::showEncouragementMessage()` signal, but first we should agree to a common way to show a non-annoying message to the users.
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