Better video thumbnails

This is a combination of a bug report and a feature request.

Currently it appears that there are two thumbnail modes.
1/ Most often there are just two thumbnails for each clip, one at the beginning, one at the end.
2/ When zoomed right in, every individual frame is shown as a thumbnail.

I would like something in between. There should always be several thumbnails. Probably the exact number
would be user configurable,, but I think choosing thumbnails so that about 2/3ths of the line was thumbnails
and 1/3 was space between thumbnails would look quite good. This would be the same no matter how
much zoom was imposed.

The bug is that currently, when you zoom right in, it seems to render all of the thumbnail. So if I have a very large
clip (20minutes) kdenlive stops responding for a few minutes while the thumbnails are rendered.