Metadata viewer: indicate that the focal length is given in milimeters
Metadata viewer: don't display invalid/unusable "focus distance" values
fix a few memleaks
cache: make stats work on legacy compiler + arch.
Merge pull request #251 from jktjkt/metadata-viewer-tweaks

Metadata viewer tweaks
Merge branch 'master' into gtk3-00-use-_key_-infix

* master:
  cache: make stats work on legacy compiler + arch.
  fix a few memleaks
  Metadata viewer: don't display invalid/unusable "focus distance" values
  Metadata viewer: indicate that the focal length is given in milimeters