It's used if there's a gtk notification daemon or application is running sandboxed without access to the freedesktop protocol.
GNotification API is poor, but should feel native on environments using GNOME technologies.
Add an experimental "small message radius" toggle.
This toggle allows switching to the pre-4.3.0, smaller message bubble
radius after an app restart. The message bubble radius styles now have
to be referenced via the Ui::BubbleRadius* and Ui::MsgFileThumbRadius*
wrappers to use the appropriate value.