Add basic ui implementation for settings to select terminal

data/org.kuchelmeister.ToolboxTuner.gschema.xml.in

@@ -1,4 +1,4 @@
-<?xml version="1.0" encoding="utf-8"?>
+<?xml version="1.0" encoding="UTF-8"?>
 <schemalist>
   <schema path="/org/kuchelmeister/ToolboxTuner/" id="@app-id@" gettext-domain="@gettext-package@">
     <key name="window-width" type="i">
@@ -13,5 +13,10 @@
       <default>false</default>
       <summary>Window maximized state</summary>
     </key>
+    <key name="terminal" type="s">
+      <default>""</default>
+      <summary>Terminal to use</summary>
+    </key>
   </schema>
 </schemalist>
+