mediafilespreviewer: Remove always-true condition
pipeline: Raise an exception when setting the timeline fails
pipeline: Simplify logging of obtained duration value
mediafilespreviewer: Consistent imports
pipeline: Keep track whether the pipeline has a duration

This fixes a stacktrace showing up when resizing clips backed by an
image. An image asset does not have a duration.
elements: Remove code with no effect
check: Remove the check for autovideosink as we switched to cluttersink
Move the modules initialization to a single place
elements: Avoid 0-sized clips to become invisible
check: Specify dependency on ClutterGst
tests: Fix the tests to initialize the modules properly