The mate-notification-properties binary had no command-line option
parsing, making it hard to know the installed version.

This adds the version flag to both mate-notification-properties and
mate-notification-daemon.

Fixes #228

Add a new 'force-display' gsettings and checkbox that, when enabled,
shows all notifications regardless of urgency, fullscreen, or
screensaver state.  Do Not Disturb still takes precedence.

Fixes #199

Style notification history rows based on urgency. Critical notifications
get a red accent bar, low priority notifications are dimmed.

The notification list popup can clip beyond the edge of the screen if
the panel applet is too close to it. This repositions the popup window
so that it doesn't go beyond the edge.

Since this applet no longer does just DND toggle, but instead allows
more comprehensive status and control, we can rename this to represent
that in a more generic way.

This changes the status icon from being two widgets to a single one and
we change the icon based on the status. The possible statuses now all
have different icons two represent all possible states in a better
semantic way:
- history on,  dnd off: user-available
- history on,  dnd on:  user-busy
- history off, dnd off: user-invisible
- history off, dnd on:  user-offline

This helps a bit with privacy. By disabling history, it wipes all
notifications in the D-Bus buffer and prevents further storage. The
history popup is also disabled.

Notification storage resumes when history is enabled.

Note that this just prevents the mate-notifications-daemon from storing
notifications, but it doesn't prevent other processes from capturing
them elsewhere.

This adds visual feedback to the notification history by showing the
number of unread notifications in the buffer.

It also adds middle-click as a DND toggle.

Adds the basis for a notification management system using the D-Bus
context to display notification history in a popup.

Fixes https://github.com/mate-desktop/mate-notification-daemon/issues/74