html_style: style selected text

Especially in chrome (75), it's very hard to see selected text in erp5
xhtml style interface, especially in listboxs.
Chrome adds a background color to selected text, but this background is
almost same color as ERP5 background.

This uses https://developer.mozilla.org/en-US/docs/Web/CSS/::selection
to make it visually clear what is selected.

/reviewed-on nexedi/erp5!889

product/ERP5/bootstrap/erp5_xhtml_style/SkinTemplateItem/portal_skins/erp5_xhtml_style/erp5.css.css

@@ -1229,3 +1229,7 @@ body[dir="rtl"] .access_tab_table td > div > ul {
   padding-right: 10px !important;
   padding-left: 0 !important;
 }
+
+::selection {
+  background: orange;
+}