Merge pull request #1 from adenexter/master

Port to Qt5
[nemo-qml-plugin-thumbnailer] Fix double free of texture.
Merge pull request #2 from adenexter/doublefree

[nemo-qml-plugin-thumbnailer] Fix double free of texture.
[nemo-qml-plugin-thumbnailer] Use a shared cache for all applications.
[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.