Address ColumnHider a11y issues

Ensure tab stop for the ColumnHider trigger node and allow it to be displayed
by pressing enter/space

* Use a button to provide tab stop and enter/space triggering for keyboard nav
* Immediately focus first checkbox within the popup when it opens

Add Dialog Role, Escape to close

* Pressing Escape while the columnhider dialog is open will cauase it to close
* Focus returns to the dialog toggle button when the dialog is closed
* Add dialog role to dialog div

Add aria-label and supporting i18n

* Add aria-label to the ColumnHider popup dialog and supporting nls
files
ColumnHider: add Keyboard mixin to first grid in test page
Fix #361: OnDemandList: empty preload node caused preload processing to stop.

In cases where the first/top preload node was visible but empty, the
second/bottom one was not being processed.  The top one had to be scrolled
beyond the buffer limit before the second one was processed.
CHANGES.md: note OnDemandList fix for reloading after resizing