libs/web: support placeholder attribute for comboboxes, fix validation quirks with input->select transition in comboboxes

libs/web/htdocs/luci-static/resources/cbi.js

@@ -441,7 +441,7 @@ function cbi_combobox(id, values, def, man) {
 	var obj = document.getElementById(id)
 	var sel = document.createElement("select");
 		sel.id = selid;
-		sel.className = 'cbi-input-select';
+		sel.className = obj.className.replace(/cbi-input-text/, 'cbi-input-select');
 
 	if (obj.nextSibling) {
 		obj.parentNode.insertBefore(sel, obj.nextSibling);
@@ -504,6 +504,10 @@ function cbi_combobox(id, values, def, man) {
 			//Do nothing
 		}
 	})
+
+	// Retrigger validation in select
+	sel.focus();
+	sel.blur();
 }
 
 function cbi_combobox_init(id, values, def, man) {

libs/web/luasrc/view/cbi/value.htm

@@ -18,6 +18,8 @@
 		-%>
 		}, '<%- if not self.rmempty and not self.optional then -%>
 			<%-: -- Please choose -- -%>
+			<%- elseif self.placeholder then -%>
+			<%-= pcdata(self.placeholder) -%>
 		<%- end -%>', '
 		<%- if self.combobox_manual then -%>
 			<%-=self.combobox_manual-%>