Don't render empty charts when heap profiling not in use

[adamgundry]

The "Heap" chart is sometimes useful even when running without heap profiling, but in that case all the other tabs display empty charts. It would be nice if the tabs were hidden and/or the page opened on the "Heap" chart.

In fact I wonder if the "Heap" tab should appear first even when there is profiling data? It gives the most general overview of the memory behaviour, and would emphasize that the other charts show only the live data (i.e. only part of the program's memory use).