FIX(a11y): Make plugin message type settings accessible

src/mumble/Log.ui

@@ -15,10 +15,13 @@
   </property>
   <layout class="QVBoxLayout">
    <item>
-    <widget class="QTreeWidget" name="qtwMessages">
+    <widget class="MultiColumnTreeWidget" name="qtwMessages">
      <property name="accessibleName">
       <string>Log message types and actions</string>
      </property>
+     <property name="accessibleDescription">
+      <string>Use up and down keys to navigate through the message types. Use left and right keys to navigate between possible notification types for a single message type.</string>
+     </property>
      <property name="alternatingRowColors">
       <bool>true</bool>
      </property>
@@ -539,6 +542,11 @@ The setting only applies for new messages, the already shown ones will retain th
    <extends>QSlider</extends>
    <header>widgets/SemanticSlider.h</header>
   </customwidget>
+  <customwidget>
+   <class>MultiColumnTreeWidget</class>
+   <extends>QTreeWidget</extends>
+   <header>widgets/MultiColumnTreeWidget.h</header>
+  </customwidget>
  </customwidgets>
  <tabstops>
   <tabstop>qtwMessages</tabstop>