Improve seeks; add one-minute binds.

[galenhuntington]

Closes #116.

• We no longer need frame counts at all so they're dropped.
• Everything is done in seconds.
• Seeks are made ten seconds exactly instead of the 400-frames approximation.
• Seeks can be done while paused and work normally.
• New key binds: shift-left/right-arrows jump by a minute.
• The new frame adjustment function is used for the stopped case; it has weird edge behavior, but then it did before too.