feat: add onPreHeaderContextMenu for Column Picker usage

[ghiscoding]

• 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)