Icons could potentially be really big in size. Instead of passing the data via
GBytesIcon (as part of the D-Bus message) allow passing a
sealed fd created with `memfd_create()`

This property allows applications to specify a sound to be played
whenever the notification is displayed. The format used is inspired by
the serialized from of GIcons.

The `markup-body` property allows applications to set markup on the
body.
The used markup is a subset of html limted to <b>, <i> and <a>.

This property allows apps to specify how the notification is displayed.

We need to hand out the activation token for XDG Activation in some way.
I think it's pretty nice that we can just add the same platform data as
used for DBus Activation to the ActionInvoked signal.

The category allows the notification server to handle specific
notification different. E.g. calls notifications.

The purpose for a button allows the notification server to style the
button specially and know the purpose of the button.

Let applications query supported options for category and button purpose.

We need to make sure that the backend supports new features introduced
to the frontend. This ensure backwards compatibility with older backend
versions.

We want to support old backends ensure we filter out properties that
were introduced in never versions.
In the case of the file-descriptor icon we can translate it to a bytes
icon to increase compatibility.

Since we now have the option to give a file-descriptor there is no need
for moving binary data across D-Bus. This also ensures backwards
compatibility by converting the icon property's `bytes` option to
the `file-descriptor` option.