prefs: Use the correct signal to monitor GtkFileChooserButton's selection changes

The correct signal is 'selection-changed', as in all file choosers.

The signal that was being used, 'current-folder-changed', only means that the current
directory being shown in the file chooser dialog has changed - not that the selection
has changed.

See https://bugzilla.gnome.org/show_bug.cgi?id=696498#c8 - although that comment
suggests the use of the 'file-set' signal, which I don't think is very useful.  It's
just a historical remnant.
Finish the introduction to the help
Add the screenshot for the help introduction
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Add help for first run of using "Web"
Add help about private browsing
Add help for browsing local files
Fix validation error in browse-private help page
Reorder the help index
Review adding bookmarks page

See bug 703365
Review help about bookmarks
Update help index

Split off a privacy section
Add help for cookie pref and data
Finish the help guide for the bookmarks dialog
Link bookmark help pages to the guide
Add help page about smart bookmarks
Add stub for bookmark topic help
Update the help index structure
Fixed typo in author's name
Fonts and user CSS pages filled out.