Context.update() implies a push(), which some of our code wasn't

accounting for, resulting in leaked state This mainly impacted

extensions, though datagrids were also doing this (not resulting in

leaks, but rather slightly slowing down rendering).
We now construct a Context as late as possible, instead of creating one

early and operating on it, and pop() when necessary.

Unit tests pass.
Tested that datagrids still work.
No longer saw any more leaks from TemplateHook.