Add logic responsible for starting web app

data/com.github.rafostar.Clapper.gschema.xml

@@ -58,6 +58,10 @@
       <default>6446</default>
       <summary>Listening port to use for incoming WebSocket connections</summary>
     </key>
+    <key name="webapp-enabled" type="b">
+      <default>false</default>
+      <summary>Run built-in broadway based web application</summary>
+    </key>
 
     <!-- Tweaks -->
     <key name="dark-theme" type="b">