Fix #1792(options): use gridOptions rather than copy them

The current arrangement is to shallow copy gridOptions, which
means changes to scalar values are not automatically noticed.

Change to reference gridOptions directly, defaulting any necessary
values over the top.  This then allows options to be referenced
and updated dynamically.
Merge pull request #1869 from PaulL1/options_watches

Fix #1792(options): use gridOptions rather than copy them