Remove unuseful instance module
tests: Segfault by re-adding an asset to the project
ruler: Make sure we are using the same values we are reporting in the log message
ruler: Rename cryptic variable names
ruler: Use the correct constants to get colors
ruler: Simplify drawTimes a bit
ruler: Clarify what params setCairoColor handles

There are colors and colors, it seems.
Passing Gdk.Color objects is not handled yet.
ruler: Highlight the different time elements

Seems easier to get an idea of the length of the displayed interval and
the zoom level.
ruler: Reuse values when we already have them
ruler: Remove unused border field
ruler: Define a constant for the times left margin
ruler: Rename some constants and more cleanup
ruler: Clarify what the spacing and scale values represent
ruler: Rename variable and reuse value

'seconds' is confusing because it actually holds nanos.
Reuse the width value of the widget.
ruler: Set the drawing color fewer times
timeline: Simplify the handler mouseleave handler
elements: Disconnect functions when cleaning up

Without this, when selecting a clip then moving the mouse over one of
the handlers then deleting the clip then moving the mouse away shows a
stacktrace because the _leaveEventCb is called.
previewers: Fix stacktrace when removing a clip

This covers a corner case, when the clip is removed just when its
thumbnailing starts.
timeline: Fix stacktrace when clicking handler

Without this, a stacktrace appears if you click a handler and then you
move the mouse away, because the finish method expects that editTo has been
called but it's not called if the mouse did not move.
pipeline: Handle negative position when pausing

This happens immediately after a render operation.