Form AJAX submit: loader, disable fields and more

**Features:**
1) loader show/hide functions in atk4_loader for more general usage
2) show loading indicator on ajax submit of form (uses atk4_loader show/hide functions)
3) disable all input elements (fields, buttons) of form while doing ajax submit
4) reenable fields (only these which was previously enabled) after ajax submit finished

**Fixes:**
1) correctly use $js_widget in Form class