Replace deprecated GTK_STOCK items

Starting GTK 3.10 GTK_STOCK items are deprecated in favor of defined labels or
standard icon names.

https://bugzilla.gnome.org/show_bug.cgi?id=720643

https://bugzilla.gnome.org/show_bug.cgi?id=722044
Remove GTK_STOCK_CANCEL icon from ignore button

There is no replacement for this icon, so better is removing the icon from the
button.

https://bugzilla.gnome.org/show_bug.cgi?id=722044