Fixes a number of issues that came up in beta 7:
- Made the text column filter work as expected when a search term is applied in the filter control (135, 144, 145)
- Added support for columns containing multiple types of data (previously this locked up the widget when sorting) 133
- Made the add row button works as expected when the grid is not sorted by the index 132
- Fixed the issue where resizing a column had a side effect of clicking the column header, which resulted in a sort being triggered 134
- Convert objects to strings before attempting to display them to fix 57
- Added new `get_selected_df` method to provide a more convenient alternative to `get_selected_rows` which reflects any filtering/sorting changes 137
- Bumped the required pandas version to address 139