Unnest thumb_job callbacks slightly
Allow calling load_video_data multiple times

Before, every call to load_video_data would start the loading
animation and add an additional three dots to the loading text. We now
only add the dots as needed, i.e., only the first time the animation
is started.
Allow customizing thumbnail seek time (part of #5)
More flexible vertical centering of loading text

Before, the height of the video was hard-coded to 180px. The text will
now be vertically centered regardless of the video height.
Generalize moving of data from controls into data attribute

We can now add more input fields to the controls without having to
touch this logic again.
Allow customizing thumbnail size (part of #5)
Make medium thumbnail size default
Give paragraphs some vertical margin
Wrap thumbnail controls in collapsible box (fixes #5)

The box can be opened and closed by clicking on the title.