do not pre-emptively generate thumbnails and waveforms when dragging
@jeff
Submitted by Jeff F.T. Assigned to Brandon Lewis @brandon_lewis
Description
When dragging a clip from the library to the timeline, it seems that waveforms and thumbnails start generating while I'm still in the drag operation (ie: have not released the mouse button).
This is undesirable because it makes dragging jerky, and will likely cause problems when dragging multiple clips at once.
Imported from https://bugzilla.gnome.org/show_bug.cgi?id=572328