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
'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.
