LP1808268: Add [disable] option to <eg-grid-toolbar-action> in eg2

LP1808268: Add [disable] option to <eg-grid-toolbar-action> in eg2

To test:
1) Apply this commit and recompile eg2.
2) Open the eg2 sandbox (https://yourdomain/eg2/staff/sandbox)
3) Ensure that the action called "Action that needs a single row"
is only enabled when one row of the grid is selected.
4) Create more <eg-grid-toolbar-actions> on grids in eg2.  Make sure
that they all enable/disable those actions per the output of the
functions you reference in the [disabled] attribute.

Signed-off-by: Jane Sandberg <sandbej@linnbenton.edu>
Signed-off-by: Bill Erickson <berickxx@gmail.com>