slidertest:accommodate shortened keys for bt msgs

... vs = Volume Set level. vg = Volume Get level.

apps/slidertest/app.js

@@ -4,7 +4,7 @@ Bangle.setLCDTimeout(0) // Easier to read the screen while developing.
 // Callbacks for use with the sliders
 
 let callback = (mode,fb)=>{
-  if (mode =="map") Bangle.musicControl({cmd:"volumesetlevel",extra:Math.round(100*fb/30)});
+  if (mode =="map") Bangle.musicControl({cmd:"vs",e:Math.round(100*fb/30)}); // vs = Volume Set level
   if (mode =="incr") Bangle.musicControl(fb>0?"volumedown":"volumeup");
   if (mode =="remove") {
     audioLevels.c = fb;
@@ -61,7 +61,7 @@ let init = ()=> {
 let audioLevels = {u:30, c:15}; // Init with values to avoid "Uncaught Error: Cannot read property 'u' of undefined" if values were not gathered from Gadgetbridge.
 let audioHandler = (e)=>{audioLevels = e;};
 Bangle.on('audio', audioHandler);
-Bangle.musicControl("volumegetlevel");
+Bangle.musicControl("vg"); // vg = Volume Get lt evel
 
 // Bangle.emit("message", type, msg);
 let trackPosition = 0;