Clears all invalid field messages in this form.
@return {Ext.form.Basic} this
function() {
        Ext.suspendLayouts();

        var me     = this,
            fields = me.getFields().items,
            f,
            fLen   = fields.length;

        for (f = 0; f < fLen; f++) {
            fields[f].clearInvalid();
        }

        Ext.resumeLayouts(true);
        return me;
    }