[Notifications] Support action-icons hint
As per freedesktop notification spec 1.2:
Supports using icons instead of text for displaying actions. Using icons for actions must be enabled on a per-notification basis using the "action-icons" hint.
Obviously this is meant for the Annotate button in Spectacle. However there is no API in KNotification to make use of this yet. Still, having the feature in Plasma means at any point once we figure out how to make Spectacle use it, it will magically work in Plasma.
Turns out, Telegram actually uses it, not sure I like the looks of it:
Edited by Kai Uwe Broulik