AdvancedDataGrid column sort can't be turned off

Hi Royale,

In mx.controls.beads.AdvancedDataGridSortBead in the mouseClickHandler when setting the column variable
event.target.data is always ~~null~~ _undefined_.
Setting the sortable property to false on a column causes this to matter.
When I changed it to event.target.element.parentElement.royale_wrapper.data _and I changed the boolean logic !col.sortable to col.sortable_ then it started working correctly.

I was going to open a PR, but my solution seems like it shouldn't work like that. I'm still trying to learn the framework, hopefully someone with more experience can explain maybe how it should work and I can make an attempt at a PR.

Start in mx.controls.beads.AdvancedDataGridSortBead, specifically mouseClickHandler, and inspect how event.target.data and the column's sortable property are used. Reproduce the issue with a column whose sortable property is false, then confirm that clicking it no longer enables sorting while sortable columns still work.

Written by the indexing model from the issue text.