Clear Filter icon sample code added which works.
I've added my own button to the main mail template (which i know will be lost on upgrades), but wanted to share as i find it's quite handy to have. Basically it's just a filter clear button that goes back to a non filter list.
I added this to /usr/local/ispconfig/interface/web/mail/templates/mail_user_list.htm as this is where I needed it most but could be done on all filter pages with element adjustments.
{tmpl_var name="filter_txt"}