Bringing the window to the foreground when it's a GtkPlug causes
the main dialog window contents to appear as a floating window. So
let's not do that.

When the device-id is selected after the window is open, switch
to the devices tab and select it.

When the settings app is launched after it's already shown, present
it rather than launching a new one.

Just create an activate action to perform the default action on
the primary instance, also don't create additional windows. We
need to make the settings dialog take an update for when device
icons are clicked in the panel plugin.

We don't want the single app logic for settings because it can be
a normal dialog and plugged into the settings manager at the same
time.

This patch adds actions to map the existing cli options. It allows
the settings dialog to be embedded as a plug and the device-id
mapping for the devices tab. Might need to rework the settings
app a little so it can active the device item rather than launch
a new window.

This changes the settings app to use GtkApplication to handle
process uniqueness rather than our custom version. Currently,
the command line doesn't get handeled properly so it doesn't plug
and the plugin doesn't popup the correct device item.

When the plugin's button is first shown (and consequently
the icon is set for it), the button isn't packed inside the
parent widget yet, consequently the GtkStyleContext is NULL.

The custom brightness notification was already dropped in
commit 5ac47c84, but this
file was forgotten at the time.

The contents of that listview are very obvious without them.

This also removed system bus "exit on close" = false.
Ideally this should be replaced by g_dbus_watch_name.

Ideally the usage of these functions is replaced by g_dbus_watch_name and
g_dbus_own_name.

Per the 4.14 roadmap, Upower 0.99 is the new minimum so drop support
for older versions.

Drop old glib and xfce4ui version checks that aren't required
anymore.

It won't support GTK3 in the LXPanel. It's suggested to either
keep using the 1.4 branch or to use the systray icon once again.

After a discussion with Mattias Clasen and exploring alternative
options, we decided to switch back to GtkImageMenuItem as it works
best for what we want here.
In order for people to be able to compile without warnings, the
relevant parts have been wrapped in G_GNUC_BEGIN_IGNORE_DEPRECATIONS.