[nemo-qml-plugin-thumbnailer] Make .nemothumbs a subdirectory of .cache.

QDesktopServices returned a path with a trailing /.  QStandardPaths
doesn't.
[nemo-qml-plugin-thumbnailer] Preserve aspect ratio when displaying thumbnails.

Not just when generating them.
Merge pull request #3 from adenexter/master

Fix display of thumbnails with fillMode: PreserveAspectFit.