WebUI: Add tooltip to regex filter button

Add a tooltip to indicate the button's function.
And change the cursor to a pointer since label is used as a button.

PR #21695.

src/webui/www/private/css/style.css

@@ -487,6 +487,7 @@ a.propButton img {
     background-size: 1.5em;
     border: 1px solid var(--color-border-default);
     border-radius: 4px;
+    cursor: pointer;
     display: inline-block;
     height: 26px;
     margin-bottom: -9px;