[DBus Runner] Add support for custom pixmap icons for results

Using the same format as notifications

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@fvogt @davidedmundson @alex @vkrause

There seems to be some kind of mismatch in formats. In KNotification ImageConverter::variantForImage it converts it to Format_RGBA8888 but in Notification plasmoid it does Format_ARGB32. It works in KNotification but somehow I find this a bit odd.

Also, ideally i'd be able to send multiple icons at different sizes/scales, but imho we'd rather have the relevant KRuner view (e.g. Kickoff with lage icons or KRunner with small icons) tell the runner about what icons size and scale it wants. But of course, icon names are preferred, and this is just for custom stuff, like favicons in tabsrunner, which are only exposed to us as one size by the browser anyway.