Add Button to Select/Deselect All Values in Filter Widget List

[DavidWittmann]

When configuring a Filter widget with the "is any of" operator you get a list of the unique values that are available for that field.

Currently, the buttons at the bottom of this dropdown menu are for 'Show all', 'Show selection', and 'Clear selection'. I propose either adding a Select/Deselect All button to these existing buttons or adding an 'All' checkbox at the top in a similar fashion as the button for the Search widget when you configure it with multiple search sources.

Adding this functionality will be essential for making it easier to quickly select all available values that are in that list of options.