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
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
timeline: Remove unuseful perspective logic
ui: Remove unused function
ruler: Use system font
ui: Remove unused class Point
prefs: Update pydoc and rename klass
tests: Move PreferencesDialog tests to test_prefs.py