Fix modifier key to keep selected item selected

Issue #1781, not using modifier key on modifierKeysToMultiSelect=true
correctly keep the selected item selected if multiple items were
previously selected
chore(Docs): Update docs for getting cell values
Merge pull request #1789 from mathiasdose/issue1781

Issue #1781: Fix modifier key to keep selected item selected
Feat(importer): create importer, bug fixes to exporter, extension to rowEdit

Importer provides the ability to import csv and json files from the file system
into your application.  If the rowEdit feature is enabled it will also save that
data to your server.

Minor edits to gridMenu to fix defaulting issues and treatment of show/hide columns
when columnDefs are present but data is not.

Extended rowEdit to have a `setRowsDirty` method that permits rows to be marked
as dirty programatically.
Merge pull request #1802 from PaulL1/create_importer

Feat(importer): create importer, bug fixes to exporter, extension to rowEdit
Fix #1800 (selection): selectionHeader should not be resizeable
Merge pull request #1804 from PaulL1/1800_resize_row_header

Fix #1800 (selection): selectionHeader should not be resizeable
Fix #1763 (selection): raise selectionChanged in more scenarios
Merge pull request #1805 from PaulL1/1763_row_selection_changed

Fix #1763 (selection): raise selectionChanged in more scenarios