Ability to clear the contents of a column

[chriddyp]

• The UI for this feature may overload the existing x button on the column or it may introduce a new icon or an additional context menu.
• Deleting the contents of a column will delete the contents across all pages (if pagination is enabled)
• If a filtering expression is applied on a column of which the contents are deleted, the filter will be cleared as well.

[chriddyp]

Similar to the comment in #318 (comment), for now I believe this is something we can override the x with. The Dash developer will be the one in control of creating a fixed UI for a very structured workflow for a very well-defined application.

In the future, we can incorporate a broader UI in order to give this option to the end-user.

[Marc-Andre-Rivet]

Discussion notes: Add a clearable nested prop to columns that has the same type as deletable -- some "eraser" icon will represent it. It will behave just like delete but will not delete the columns. To be improved in #495.

[Marc-Andre-Rivet]

The current column.deletable action implementation prevents the x from being displayed if the pagination is custom (not controlled by the table), the current implementation of column.clearable will use the same limitation.

If a filtering expression is applied on a column of which the contents are deleted, the filter will be cleared as well.

This is not being done for column.deletable right now. As this is implemented for column.clearable, it should behave the same for the other.