fix: align images in notification center at top

Set the images in notifications in the notification center to be top
aligned, instead of vertically centered.

Closes phuhl#119.

notification_in_center.glade

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<!-- Generated with glade 3.38.2 -->
+<!-- Generated with glade 3.40.0 -->
 <interface>
   <requires lib="gtk+" version="3.24"/>
   <object class="GtkAdjustment" id="adjustment1">
@@ -14,7 +14,7 @@
       <object class="GtkImage" id="img_img">
         <property name="visible">True</property>
         <property name="can-focus">False</property>
-        <property name="valign">center</property>
+        <property name="valign">start</property>
         <property name="margin-start">10</property>
         <property name="icon_size">0</property>
         <style>