synchronize GET /imagery? remote control

[A_Pirard]

Imagery layers can exist if no "Data Layer" is open (open data, open imagery, close data).
But GET /imagery? remote control will return code 4 if no data layer (window) is open.
Furthermore, I could not find a nondestructive control to determine if such a window is open.
So, either GET /imagery? should work in a no-data-window state, best option,
or some read-only control, e.g. /version? should tell when it can work.
This would allow a script to open favorite imagery at JOSM startup.

Another issue is that imagery starts in the Show state.
When their number is high, starting in Hide state would be appreciated.

[Don-vip]

In 5837/josm:

fix #8586 - Remote Control: allow imagery handler to work without any mapFrame + add wiki link in HTTP 400 error message

[A_Pirard]

It works great! Thanks!
Just that sometimes JOSM becomes unresponsive while executing the second wget in josm-layers script.
Its window is all white and it has to be killed.
Difficultly reproducible and I didn't see it happen if an extra sleep 1 is added.

JAVA_OPTS="-Xmx738M" josm &
for ((t=0; t < 120 ; t++)); do
# looop until JOSM remote control port opens
  wget -nv -O- "localhost:8111/version?" && break
  echo remote control attempt after $t sec returned code $?
  sleep 1
done
sleep 1
josm-layers