[form] fix #719285, due to multiple calls to restore_previous_post, by proper refactorings * move __init__ code from FieldsForm to Form. Must behaviour here should actually be in the Form base class * avoid buggy duplicated call to restore_previous_post * move some code that was in the form renderer to the form'__init__ method (__redirectpath & __form_id hidden input handling)) * 'formvid' should now be specified on form selection, not on form rendering

function set_tab(tabname, cookiename) {
    // set appropriate cookie
    asyncRemoteExec('set_cookie', cookiename, tabname);
    // trigger show + tabname event
    trigger_load(tabname);
}