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.
elements: Keep the handlebars hidden when the clip is selected
elements: Try a new model for the _create methods
elements: Change the border color when a clip is selected