Add a clock to show the position of the edit, to remove ambiguity
If a Range is selected, pre-fill the position and length clocks
Clock format should default to the editor's secondary clock mode
Pass 'true' to accept_on_focus_out of AudioClock, so that editing the
insert/remove time clock and then clicking elsewhere without hitting
<Enter> first doesn't mysteriously set the time back to 00:00:00.000.
'Cut time' implies that the cut timeline items might end up on the
clipboard to be pasted somewhere: this isn't the case at present, so rename
the functions, class, &c. to say 'remove' rather than 'cut'.
Rename insert_time_dialog.{cc|h} to insert_remove_time_dialog while we're
at it.
