adding new events onPreHeaderContextMenu and onPreHeaderClick which is mostly to be able to open the Column Picker from either the regular column headers OR the Column Group from the Pre-Header when defined
also works with/without frozen column
fixed some examples where the preheader weren't recalculated after reordering columns or hiding columns from picker
Note: before calling the column picker, we need to make sure that the pre-header target element is in fact a column group, because if it's anything then we shouldn't open the column picker (e.g. draggable grouping dropzone)
onPreHeaderContextMenuandonPreHeaderClickwhich is mostly to be able to open the Column Picker from either the regular column headers OR the Column Group from the Pre-Header when defined