Appearance: Set matching Xfce4-notifyd theme if there is one.

4.18 allows convenient setting of both the GTK 3 and Window Manager themes (really appreciate it).

So, here's for doing the same to Notifications:

xfconf_channel_set_string (xfconf_channel_get ("xfce4-notifyd"), "/theme", name);

This would be useful particularly for light/dark switches.

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Also, I think the wording on the option could be a little bit friendlier. For instance:

Set matching Xfwm4 window manager theme if there is one

Though applying this change to the tree will cause some row overflow.