Use touch events to sync the range element's value when the bar is touched.
This means you no longer have to touch the tiny ball on the seek bar on iOS.
Fixes#1918
This factors out common functionality and structure from the volume
bar and seek bar, both of which involved specific CSS workarounds for
IE. The seek bar had logic to keep the value from jumping around
during casting, which now the volume bar benefits from, as well.
Finally, the seek bar code was spread out throughout controls.js, and
now it has its own class.
Closes#1913
Change-Id: I299476ccbc27f28f7b225a6e6f8b0d21abe5baf2
Matthias
Joey Parrish