Add "All" to Single Selection slicer

[Robert_Jensen1]

Single-selection slicers should have the explicit option of "All" in addition to the unique values of a column. This makes for a more consistent experience. Currently, if a slicer has two options and both are checked, the slicer reads "Multiple selections." If the user checks the 'Select all' option when single-selection is not enforced, then the slicer reads "All."