Playhead position change only on click release.
And only if the user was not dragging.
Removed unecessary drawing call during sync syncPlayhead.
Initialised playheadPixel value.

- fixed playhead pixel display after dragging with no playback and no seeking
Tweaked dev canvas height.

And thinking to have that "per view".
Playhead animation is no more bound to the frame seek.

Basically:
- playhead position is calculated anyway, displaying it or hiding it if necessary
- `newFrame` calculation is going to be performed on playback heartbeat rather than on playhead progress
Frame seeking on playback.
Fixing JSHint issues.
Merge pull request #25 from bbcrd/fix-25

Dragging zoomview bug (low priority)