Implement in-app notifications as App wide actions.

At first I tried creating custom InAppNotifications and send
them to the main loop to be added. That does not work sicne gtk
widgets are not thread safe. For now we can try having Action messages
that create them on demand. As oppose to create first then pass them.

hammond-gtk/resources/gtk/inapp_notif.ui

@@ -2,7 +2,7 @@
 <!-- Generated with glade 3.21.0 -->
 <interface>
   <requires lib="gtk+" version="3.20"/>
-  <object class="GtkOverlay" id="inapp_notif_overlay">
+  <object class="GtkOverlay" id="notif_overlay">
     <property name="visible">True</property>
     <property name="can_focus">False</property>
     <property name="halign">center</property>

hammond-gtk/resources/resources.xml

@@ -9,6 +9,7 @@
       <file preprocess="xml-stripblanks">gtk/shows_view.ui</file>
       <file preprocess="xml-stripblanks">gtk/shows_child.ui</file>
       <file preprocess="xml-stripblanks">gtk/headerbar.ui</file>
+      <file preprocess="xml-stripblanks">gtk/inapp_notif.ui</file>
       <file compressed="true">gtk/style.css</file>
   </gresource>
 </gresources>