Bug 769112 - The metadata windows takes a lot of CPU.

The new metadata viewer is based on a version of the old metadata
plugin that still contained this bug, and a few other bugs that've
been fixed since then.  Reapply those fixes to the new plugin.

This is essentially an adaptation of commits
f8e291bf,
ce9e7fea,
38c79600,
and 801bd8fb.