Add a "CountIn" also in audio export

This is going too far for me. If user wants a count-in in the audio rendering, they can add an instrument and a measure of quarter notes.

See also #154666: Add metronome sound when exporting to audio (mp3, wav, etc.)

fwiw; when I want audio count-in in my MP3s or MIDI exports I add an instrument I label 'count.' I created a SF2 soundfont that has samples of me actually counting - "one" (D4), "two" (E4), "three" (F4), etc. So what I do is add extra measures at the beginning of the score, and put my counting notes in there. I also usually add repeats, as necessary, for the number of verses I wish to have in the audio - start repeat after the count-in measure (or multiple measures really for some beginning silence, and perhaps a measure + pick-up worth of counting). On the ending repeat you can go into measure properties and set the replay count if wishing more than two. Of course, more complex scores may require more subtle rewinding/repeat schemas to get what you need in the audio. So far this all worked fine for me. Of course, it would be great if all this could be automated; tho I personally would have greater enhancement priorities.