Export current view

[jchuahtacc]

Added export_view function to QGridWidget to export all data currently being viewed (excluding any rows that have been filtered) to a new dataframe.

Also added export_all which exports all rows, regardless of filtered status.

Both functions take a callback, which will be passed a dataframe with requested rows. Indexes are not preserved.

This is in response to Issue #87 - Feature Request: Return visible rows when grid is filtered

[fhgd]

In addition to exporting the filtered rows is it possible to export the selected rows too?

[jchuahtacc]

There is already a get_selected_rows method that returns an array of indexes of selected rows. I ended up closing this PR and opening a new PR that uses the same type of logic to get indexes of rows after filtering. It's also worth noting that changes made to the data in the grid are reflected in the _df member, though there is no accessor method for it.