Update the notification style according to the latest mockups:
- make notification icons smaller and prefer symbolic variants
- remove background box when not hovered/focused
- increase spacing between elements
- use normal text sizes
https://bugzilla.gnome.org/show_bug.cgi?id=775763
Currently the secondary actor (if set) and the close button are
exclusive, that is the latter replaces the former on hover. As
the swapping feels rather busy and there's no real reason both
cannot be shown at the same time, keep the secondary actor always
visible.
A welcome side effect is that it no longer needs to be placed at
the end, so we can move the notification timestamp right next to
the corresponding title.
https://bugzilla.gnome.org/show_bug.cgi?id=775763
The section titles usually don't provide a lot of value - messages
themselves are usually pretty unambiguous about their type, and
having a hidden shortcut to some settings panel or application isn't
essential either - except when showing the selected date when browsing
other days, as it adds context to the listed events. Based on that,
remove the section title as a general MessageListSection feature and
move it into the EventsSection, where we only show it when it is useful.
https://bugzilla.gnome.org/show_bug.cgi?id=775763
The latest mockups contain a button to clear all sections at once.
As some elements cannot be cleared, we need to provide that information
to avoid offering an action that has no effect.
https://bugzilla.gnome.org/show_bug.cgi?id=775763
We lost media controls outside of notification banners when
implementing the new notification designs. Reimplement this
functionality as a dedicated "Media" section in the message
list based on MPRIS.
https://bugzilla.gnome.org/show_bug.cgi?id=756491
Currently both the base classes for messages/sections and the message
list itself that instantiates the available sections are located in
the same module. As a result, it isn't possible to define sections
in a different module without introducing circular dependencies. The
Calendar module is already unwieldily large, so split it up a bit to
avoid it growing even bigger in the future.
https://bugzilla.gnome.org/show_bug.cgi?id=756491
