Feature/375 time axis horizontal zoom

[2cylu2]

Adds a time axis and updates horizontal zoom to use fixed time scaling (pixels-per-second) for consistent behavior across media files.

[cwitkowitz]

The time strip looks great! I seem to be encountering an issue though: after zooming in all the way, vertical scroll no longer controls horizontal zoom but instead horizontal panning (however only on a subsection of the display region). Also, I think the maximum zoom should probably be such that ~5 seconds are displayed, rather than ~1 minute. Otherwise great start!

[2cylu2]

• Updated time axis to use adaptive major/minor tick scheme with improved labeling and visual clarity
• Adjusted horizontal zoom to allow zooming in to ~5 seconds while preserving full-track zoom-out behavior
• Improved scroll interaction so that when zoom limits are reached, scrolling pans instead of doing nothing