Look for help-specific images in the new images_helpimg archive

This requires distinguishing between 2 kinds of images: internal to
help (in helpcontent2/media/ and helpcontent2/res subfolder) and external
images from core repo (mostly icons) referenced in help

Change-Id: If1d639c255fb802c3ee7d7eab10fc0a84324f335
Reviewed-on: https://gerrit.libreoffice.org/31417Reviewed-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>
Tested-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>
Tested-by: Jenkins <ci@libreoffice.org>

vcl/source/image/ImplImageTree.cxx

@@ -264,6 +264,8 @@ OUString ImplImageTree::fallbackStyle(const OUString& rsStyle)
         sResult = "galaxy";
     else if (rsStyle == "sifr" || rsStyle == "breeze_dark")
         sResult = "breeze";
+    else if (rsStyle == "helpimg")
+        sResult = "";
     else
         sResult = "tango";
 

xmlhelp/util/main_transform.xsl

@@ -111,6 +111,7 @@
   <!-- parts of help and image urls -->
 <xsl:variable name="help_url_prefix" select="'vnd.sun.star.help://'"/>
 <xsl:variable name="img_url_prefix" select="concat('vnd.libreoffice.image://',$imgtheme,'/')"/>
+<xsl:variable name="img_url_internal" select="'vnd.libreoffice.image://helpimg/'"/>
 <xsl:variable name="urlpost" select="concat('?Language=',$lang,$am,'System=',$System,$am,'UseDB=no')"/>
 <xsl:variable name="urlpre" select="$help_url_prefix" /> 
 <xsl:variable name="linkprefix" select="$urlpre"/>
@@ -119,9 +120,9 @@
 <xsl:variable name="css" select="'default.css'"/>
 
 <!-- images for notes, tips and warnings -->
-<xsl:variable name="note_img" select="concat($img_url_prefix,'res/helpimg/note.png')"/>
-<xsl:variable name="tip_img" select="concat($img_url_prefix,'res/helpimg/tip.png')"/>
-<xsl:variable name="warning_img" select="concat($img_url_prefix,'res/helpimg/warning.png')"/>
+<xsl:variable name="note_img" select="concat($img_url_internal,'media/helpimg/note.png')"/>
+<xsl:variable name="tip_img" select="concat($img_url_internal,'media/helpimg/tip.png')"/>
+<xsl:variable name="warning_img" select="concat($img_url_internal,'media/helpimg/warning.png')"/>
 
 <!--
 #############
@@ -800,6 +801,9 @@
 <xsl:template name="insertimage">
   <xsl:variable name="src">
     <xsl:choose>
+      <xsl:when test="starts-with(@src,'media/')">
+        <xsl:value-of select="concat($img_url_internal,@src)"/>
+      </xsl:when>
       <xsl:when test="not($ExtensionId='') and starts-with(@src,$ExtensionId)">
         <xsl:value-of select="concat($ExtensionPath,'/',@src)"/>
       </xsl:when>