The fix for taskbar tooltips in 6a2d214b55 was incomplete: it causes the label
to contain escaped titles. Use set_markup so that GTK decodes markup again,
but only if requested by the user (disabling markup is needed if using format
strings like "{title:.15}" to avoid terminating the string in the middle of an
XML entity).
If there are multiple icon themes defined in the config option
'icon-theme' the module will try from left to right to find an icon.
The system default will always be added to this list.
