improve code of columnChooser

Add `dialogClass: "ui-jqdialog"` option to the dialog to have correct size of font independent on the current `em` size.
Use internal `data` of the multiselect widget to access all internal elements.
Prefer to use `$.ui.multiselect.prototype.widgetFullName || $.ui.multiselect.prototype.widgetName` instead of `"ui-multiselect"` or `"multiselect"`.
Fix cut&pased error
Add .ui-jqgrid-hdiv { overflow-y: hidden; }
Merge pull request #644 from OlegKi/master

Improving columnChooser, fixing overflow-y of hdiv