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
controls: Make sure the is_audio field is always set
timeline: Fix the scrollbars alignment
elements: Reuse the cursor objects

Also use LEFT_PTR which is the normal mouse pointer instead of ARROW.
previewers: Use a constant for the audio previewer height
widgets: Add some padding to the zoom slider

..so it does not touch the ruler.
misc: Cleanup path_from_uri
Make sure autogen uses python2