Dash-table

Fixed
- Fix double click regression [63](https://github.com/plotly/dash-table/pull/63)

Added
Treat empty strings as none
- sorting_treat_empty_string_as_none takes value True or False
Overrides sorting default behavior to consider empty strings as a nully value.
Note: This is a stopgag prop, full implementation of sorting overrides will most probably deprecate it.
Default value is False.

Fixed
setProps bug fix
- Fixing initialization issue where the FE wrongly believes it's working in DEV mode

Added
Additional `sorting_type` prop that can take value 'multi' or 'single'
This prop defines whether the user can sort based on multiple columns or can only sort by one column at a time. The default value is 'single'.

Added
Consolidating virtualization, sorting, filtering
- First steps to make sorting work from both FE and BE *
- and consistent with Virtualization settings *

New Props
- sorting -> ['fe', 'be', true, false] (default: false) -- replaces `sortable` prop
- sorting_settings -> array of { field, ascending } -- replaces `sort` prop
- `virtual_dataframe` (READONLY)
- `virtual_dataframe_indices` (READONLY; not officially supported yet -- IN DEVELOPMENT)

virtual_dataframe vs. dataframe
- the virtual dataframe is the content of the viewport for the user (e.g. user has a 10k rows dataframe with FE/250 lines paging, on 1st page -> the `virtual_dataframe` contains items [0,250[ of the dataframe); the dataframe still contains 10k items
- 10k rows, no paging, sorting and filtering -> the virtual dataframe contains items visible in the viewport, in the visible order; the dataframe still contains 10k items
- if the user modifies a cell, the dataframe and the virtual_dataframe are updated with the new data

Deprecated
- sortable
- sort
- dataframe behavior on sort (see below)

Added
New props for Conditional Style, Conditional Dropdown, Filter
- filtering -> ['fe', 'be', true, false] (default: false)
- filtering_settings -> AST query string (default: '')
- column_conditional_dropdowns
- column_static_dropdown
- column_conditional_styles
- column_static_style
- row_conditional_styles
- row_static_style

Deprecated
- column style
- column options
- dropdown_properties prop