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
Remove mainwindow's fullscreen feature

With all the efforts I put into making it work, it breaks my heart to remove it.
However, with the planned use of client-side window decorations with HeaderBar,
it makes no sense to keep it.

First, the HeaderBar saves a ton of space, making fullscreen much less needed:
it takes 48 pixels instead of the previous 101 pixels.

Second, the way the fullscreen feature worked was by ripping out the
main toolbar, adding it on a temporary floating widget, then doing the reverse
operation when exiting fullscreen. This approach won't work with the HeaderBar,
as you cannot unset the client-side titlebar decoration, and the buttons we put
in HeaderBar are not contained in a GtkToolbar widget (themes don't like it).
Manually ripping out each and every widget from the HeaderBar isn't an option.

Removing this also reduces code complexity and problems (such as dealing with
detachable UI components or multiple monitors).
viewer: Add a button to undock the viewer directly instead of using menu actions
mainwindow: Move the switchContextTab method to the proper section
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.
mediafilespreviewer: Make the previewer work with cluttersink
viewer: Make the AspectFrame part of the ViewerWidget