Pivot table default filter (#508) ...

* Make pivot table shared strings internal only

* Clean up TestHelper

* Save selected values in filters

* Implement DateTime type in saving pivot tables

* Implement loading of pivot table filters

* Update pivot table examples

* Implement filters with multiple selected values

* Update reference pivot table file

Implement pivot table row and column header captions

Skip pivot table validation test for XLConstants.PivotTableValuesSentinalLabel

Check pivot table source range headers before adding field / value

Fix pivot table saving

Unknown changes - something to do with line endings?

Extend API to collapse pivot field (all items in field, not items individually)

First stab at loading pivot tables

Fix pivot table custom number format

Save pivot table value formats correctly

Add overrides to correctly check equality

Rename "Pct*" variables and values to "Percentage*", because I don't like abbreviations.

Use customName as dictionary key, because we could have multiple pivot table values with same source.

Pivot table improvements: add count attribute (to be in line with what Excel does). ...

Add sentinal value for "Values" column and row label.

Project hierarchy cleanup