plugin api: Add audio_pre_ff_rewind to plugin's API

According to wps code audio_pre_ff_rewind function should be called
before any rewinding. It stops playback and automatically resumes it
after audio_ff_rewind call

So, let's add audio_pre_ff_rewind to plugin's API

Lua scipts were tested:
```lua
-- has issue with rewinding
rb.audio("ff_rewind", 0)
```

```lua
-- no issue with rewinding
rb.audio("pre_ff_rewind")
rb.audio("ff_rewind", 0)
```

Change-Id: I2ad6b9c396760b2086bc0a28633a1c80c3512739

docs/PLUGIN_API

@@ -89,6 +89,10 @@ struct mp3entry* audio_current_track(void)
     \description
     \see [S[firmware/export/id3.h]]
 
+void audio_pre_ff_rewind(void)
+    \group playback control
+    \description prepare playback for rewinding. Playback will be paused
+
 void audio_ff_rewind(long newtime)
     \group playback control
     \param newtime