New "seekable" states, range styling, and fixes

• feat(seekable-ranges): Add media-seekable/media.seekable state support. Update media-time-display and media-time-range to handle seekable. 5fe51ce
• fix: conslidate and simplfy selectors dealing with pointer-events (#222) 575a8f2
• fix: remove stale thumbnail preview data when no thumbnails are longer visible (#219) f8bbd85
• Add custom translate CSS var for input range track (#216) 5a9630f

v0.6.3...v0.6.4