Fix notification and stopwatch empty states

Previous solution was relying on fomantic selector `.ui.label.hidden` to
hide the elements in their empty state, but this doesn't work any more
with the removal of the `label` class. Instead, introduce a standalone
CSS rule for the `hidden` class, which is universally usable as a single
class.

We can unfortunately not use the existing `hide` class because without the
`!important`, it does not have enough specificity to win against
fomantic's `.ui.menu:not(.vertical) .item {display: flex}` rule.

Author: silverwind

web_src/less/_base.less

@@ -1815,6 +1815,11 @@ footer {
   }
 }
 
+/* same class as used by fomantic ui, but as a standalone selector */
+.hidden {
+  display: none !important;
+}
+
 .center:not(.popup) {
   text-align: center;
 }