Topic hierarchy: Use display titles

Now display titles are respected when drilling down the content
hierarchy.
Barcode scanner: Don't display multiple dialogs

If mGuideid == -1 that means that there was an error during the guideid
parsing so we don't want to try to fetch guideid -1 and display another
dialog.
Merge pull request #168 from iFixit/display-titles-in-hierarchy

Topic hierarchy: Use display titles
Merge branch 'master' into themes-cleanup