Allow trimming story video and music with screen readers - #2160

A video put in a story is cut to the part that goes out by dragging the
handles at either end of the strip of frames under it. The strip and
its handles are drawn by hand and moved by a finger alone, so a screen
reader found nothing there at all, and a video could only be posted
from its start, cut wherever the longest a story allows ran out.

Music added to a story is cut the same way, by the handles at the ends
of its own strip, and was just as closed.

The start and the end of the part kept, of the video and of the music
alike, are places of their own now, each read with the time it stands
at, and moved a second at a time by swiping up or down, as a slider is.
They are held to the same limits a drag is: never closer than the
shortest part allowed, never further apart than the longest, the music
never outside the video it plays over, and the other end follows where
a drag would move it.