Improvement of Custom library editor

[PetrS]

Custom library editor was improved for handling bigger libraries, specially for EC Annexes

• Category and item columns are always visible and cannot be horizontally scrolled away
• Random reordering of rows by click on the header of column was removed. It was main problem with Struct type of variables. Ordering of rows is possible only by Category and Item.
• Reordering columns by Drag & Drop by its header is now correctly saved.