app: add --show-debug-menu command-line option

The debug menu is currently not included in stable versions.

Include the menu unconditionally, but hide it, and its associated
actions, by default in stable versions.  Allow enabling the menu
using a new --show-debug-menu command-line option, in the same vein
as --show-playground.

menus/image-menu.xml.in

@@ -23,7 +23,7 @@
         <menuitem action="dialogs-document-history" />
       </menu>
 
-<!--  The debug-menu is automatically excluded for stable releases  -->
+<!--  The debug-menu is hidden by default for stable releases  -->
       <menu action="debug-menu" name="Debug">
         <menuitem action="debug-mem-profile" />
         <menuitem action="debug-benchmark-projection" />

menus/menus.xsl

@@ -38,12 +38,6 @@
     <xsl:apply-templates />
   </xsl:template>
 
-  <xsl:template match="menu[@action='debug-menu']">
-    <xsl:if test="$unstable-menus='yes'">
-      <xsl:call-template name="identity" />
-    </xsl:if>
-  </xsl:template>
-
   <!-- need to strip the XInclude namespace declaration from the ui element -->
   <xsl:template match="ui">
     <ui>