previewers: Fix stacktrace when removing a clip

Queued clip previewers which are waiting are removed from the queue and
everything is fine, but if the PreviewGeneratorManager starts to work on
a previewer and its clip is removed from the timeline, the previewer will
unset its pipeline attribute and an exception occurs.
timeline: Fix stacktrace when clicking handlebar

Without this, a stacktrace appears if you click a handlebar 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: Initialize the border attribute in the constructor
elements: Change the border color when a clip is selected

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=635987
controls: Make sure the is_audio field is always set
timeline: Fix the scrollbars alignment by using Gtk.Grid
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
elements: Remove margins between clips

The 1px margins can be caused by rounding errors.
bin: Add --help to the dev script
bin: Remove an if statement
ruler: Use a smaller font for the millis
ruler: Clarify what constructor parameter is
ruler: Show the hours and millis only if useful
ui: Remove unused function to draw rounded rectangles in Cairo