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: Remove unuseful default label widget value
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
ruler: Remove hardcoded background color
timeline: Combine the setting of the timeline and of the project
viewer: Clarify that the pipeline param is not None
viewer: Rename PitiviViewer to clarify what it represents
viewer: Make the undock_action param mandatory
viewer: Make target a property to clarify what it represents
timeline: Remove unused togglePlayback method
viewer: Remove generic try/catch
viewer: Preview only UriClip trimming
viewer: Get rid of the ambiguous _tmp_pipeline field