While the notification spec allows tags like <a href="...">...</a> and
<img src="..." alt="...">, pango cannot parse these tags and therefore
these tags should be removed before passed to pango.
Also the method notification_extract_markup_urls is not needed anymore,
as markup_strip_a can return URLs optionally.
This implies, that URL replacement is now indicated via show_indicators
for URLs and the dmenu string is in the format of
'[text between a tags] URL\n'. This is similarly handled for images,
too.
