fixing .el with undefined

See the bug fix https://github.com/tonytomov/jqGrid/pull/663 posted by
Vitalii Kirilvit
test for existence of disableSelection, limit scope of usage it

The method `disableSelection` exists starting with jQuery UI 1.6, but
it's declared as deprecated since jQuery UI 1.9 (see
[here](http://jqueryui.com/upgrade-guide/1.9/#deprecated-disableselection-and-enableselection)).
The limit of scope of usage of disableSelection are described by Vitalii
Kirilvit in https://github.com/tonytomov/jqGrid/pull/664