Pandas loader to work correctly with Index

[vladignatyev]

Currently, the from_dataframe generates a table from the simple dataframe properly.

But when we add a custom index column to the dataframe, the ipysheet does not respect the index when we click column header for sorting.

Current behaviour: 1) When the user clicks a column header, the ipysheet sorts the whole table, missing the correct sort order in the index column. The index column looks like frozen. 2) The width of the index column is fixed and do not resize following the content width, neither we have a resize handle for that, despite the handle for every other column.

Desired behaviour: 1) When the user clicks a column header, the index column react to the sorting and its order get matched the row order in the data after sort. 2) The width of the index column is not fixed and matches the content inside. Also, the GUI for the index column works consistently and allows user to resize the index column as a normal column.