lock the UI to prevent resorting the item list

[rich-coe]

I removed the ability to sort the items in the item list. I did this because I would accidentally click in the header bar and the item list would sort, something that I didn't want it to do and it took a long time.

This request would add the UI feature to lock the header bar to the desired sort order and not react to mouse clicks in it until it was unlocked.

[lwindolf]

I believe switching from GtkTreeView to GtkListBox will automatically solve this. Then there won't be a header anymore. Sorting needs to be controlled in a different way then.