Using the (tweaked) annotation feature in waveform-playlist.

Otherwise spacebar does not work if focus is on the sound input or a
bookmark link (in development). We will need to revise the approach if
we make any text input fields.

Unfortunately not as easy for the borders of the fragment selection; for
now just making those stay 'inside' the selection (through box-sizing).

E.g. 3-6 is not descriptive enough. Better to write 0:03–0:06.