mainwindow: Replace the menubar and main toolbar by HeaderBar and MenuButton

Fixes bug #708375
Depends on GTK+ 3.10

This commit removes any dependency on GTK actions for the main window. It also
changes translatable strings and moves the "screenshot" feature to mainwindow.
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.
dogtail: Fix some
mainwindow: Allow Ctrl-Q to close the app
Rename variables with short names
viewer: Fix the trim preview to work with the clutter sink
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.
pipeline: Fix _getDuration to return the value
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: Fix the PreviewWidget expansion
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