Normalize negative indices.
Paint Data widget refactor/rewrite.

Now using pyqtgraph instead of orangeqt.
Added some docstrings.
Merge pull request #109 from ales-erjavec/paint-data

Paint data widget rewrite
Group lines in parallel coordinates.
Use logger instead of prints.
Fix display of curves in parallel coordinates plot.
Move imports to top of the file.
Merge pull request #111 from astaric/pc

Show LAC clusters in parallel coordinates
Fix outdated context manager.
Remove debug code from _execute_sql_query.
Merge pull request #114 from astaric/fix-setting-handler

Fix outdated context manager.
Approximate length estimation for SqlTable.
Change _execute_sql_query to a context manager.

It now also uses a connection pool instead of a single connection to
allow background queries performed in parallel.
Do not allow filters with no conditions.
Avoid creating empty filters.
Use approximate data set sizes in Select Rows widget.
Update Data Info with exact size once it is computed.
Do not apply row filters twice.
Always use the same approximation for length.