webhelper: Add support for string list settings
webhelper: Add support for basic bookmarks
webhelper: Fix a crash on GTK < 2.24

GTK < 2.24 had a bug leading to a crash when setting the tooltip of an
entry icon before the icon itself.  Work around it by setting the icon
first.

See https://bugzilla.gnome.org/show_bug.cgi?id=606103
webhelper: Add bookmark completion to the URL entry
Merge pull request #179 from wip/webhelper

webhelper: Add support for basic bookmarks