Qt: change ownership of thumb fetcher to infos frame

So that it can be shared between all info widgets.

Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
Add warning when failing to fetch thumbnail

Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
Qt: Display emission thumbnaill

Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>