viewer: Get rid of the ambiguous _tmp_pipeline field
viewer: Remove code which never executes
viewer: Get the seeker instance once
viewer: First commit to prove the cluttersink concept.
viewer: Make the cluttersink play nice
viewer: Improve debugging messages
viewer: Focus back the timeline when using the buttons

This means the button will lose focus when it's activated. The goal is
for the timeline actions to remain enabled, as they are disabled when
the timeline or ruler loses focus.
Rename variables with short names
pipeline: Remove obsolete constructor parameter
mediafilespreviewer: Get rid of unuseful time_format field
pipeline: Make sure each pipeline has a cluttersink

Also make it easier to connect a pipeline to a ViewerWidget.
mediafilespreviewer: Make the previewer work with cluttersink
viewer: Make the AspectFrame part of the ViewerWidget
mainwindow: Get rid of generic try/except when previewing
mainwindow: Move the preview window size logic to a new method
mediafilespreviewer: Keep the PreviewWidget size fixed
mediafilespreviewer: Remove always-true condition
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.
check: Remove the check for autovideosink as we switched to cluttersink