Show previews when importing from memory card: Closes #2959
Show previews when importing from memory card: Closes #2959

Uses fallback code to load metadata through alternate means (i.e.
GVFS) when unable to load through GPhoto itself.

abrauchli/shotwell,abrauchli/shotwell,ritchiewilson/shotwell,ritchiewilson/shotwell,jimbonator/shotwell,jimbonator/shotwell,abrauchli/shotwell,ritchiewilson/shotwell,jimbonator/shotwell