Allow widgets to report fruitless invocations

[dmbaturin]

For performance reports mentioned in #75, it would be nice to know how many times widgets were executed on pages where they didnt't actually have any work to do, so that those pages could be safely excluded.

Trying to track that by diffing element trees is likely to create performance problems. However, built-in widgets can report that, and plugins can opt into reporting fruitless invocations.

A plugin may be able to set a global, like:

fruitless_invocation = 1

then those counts would be included in performance reports.