Change previous chapter button to follow more common behaviour

[martinconolly]

Feature request

Chapter back should return to the start of the current chapter on first press.

Currently, pressing < mid-chapter jumps straight to the start of the previous chapter, so if you're partway through chapter 35, one press takes you to chapter 34.

The near-universal convention across audiobook and chapter-aware media players is:

First press → start of the current chapter
Subsequent presses (within a short grace period, typically 2–3 seconds) → step back through previous chapters

[GianniCarlo]

@martinconolly this will ship in the next release, thanks for pointing this out