Install `onClick` event only for table headers

[alexandru-dinu]

Current implementation:

function onClick(evt: MouseEvent): void {
    const htmlEl = (<HTMLInputElement>evt.target);
    
    const th = htmlEl.closest('thead th');
    
    // click was not performed in a table, exit
    if (th == null) { return; }
    
    // click was performed in a table
    // ...
}

Maybe it's better to do this rather than registering onClick events for all table headers in the current page?