Use maps internally for dashboard panels in jsonnet library

[malcolmholmes]

When we render Jsonnet for adding panels to dashboards, we add a panel to a list. In Jsonnet, this makes it difficult to later interact with those dashboards.

There's a simple trick that can solve this: have the addPanel method add the panel to a hidden map, and add its name to a hidden field order list. Then, render the actual panel list from these with a simple for loop.

This way, the dashboard can later be amended by patching the panel map without the need for iteration as would be needed if it were a list.