inspector: Add a first-time warning

With the keybinding, it is possible that users may trigger the
inspector unintentionally. Show a dialog that informs them about
whats going on and gives them a chance to back out.

The warning dialog can be bypassed with the
org.gtk.Settings.Debug inspector-warning setting.

gtk/org.gtk.Settings.Debug.gschema.xml

@@ -11,6 +11,14 @@
         the keybinding are Control-Shift-I and Control-Shift-D.
       </description>
     </key>
+    <key name='inspector-warning' type='b'>
+      <default>true</default>
+      <summary>Inspector warning</summary>
+      <description>
+        If this setting is true, GTK+ shows a warning before letting
+        the user use the interactive debugger.
+      </description>
+    </key>
   </schema>
 
 </schemalist>