timeline: Move the toolbar logic to TimelineContainer

Also remove the standalone toolbar logic, to simplify the code.
timeline: Move UI data to xml file
timeline: Make the timeline toolbar and scrollbars background transparent
timeline: Rename for consistency
Use Gtk STYLE_CLASS constants for add_class
timeline: Be more strict when the project is loaded
cleanup
timeline: Access the pipeline through the project field
timeline: Fix the retrieval of the Pipeline state
widgets: Hide the zero hour in the time widget
viewer: Add spacing around the play buttons
elements: Remove unused key-press-event signal handling
viewer: Disconnect other keyboard events handlers when time widget focused

For example when pressing left/right to edit the text, the playhead will
not move anymore.
widgets: Move function to the class where it's used
widgets: Move the tests to a test file
widgets: Simplify DefaultWidget
widgets: Make the properties parameter mandatory
ruler: Fix the play header display

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=721972
ruler: Use the same color for the entire playhead
ui: Move color function to ui