feat: use any source for expo/weight/offset (#5220)

Co-authored-by: philmoz <phil.a.mitchell@gmail.com>
Co-authored-by: 3djc <3djc@gh.com>
Co-authored-by: elecpower <elecpower@users.noreply.github.com>

radio/src/strhelpers.cpp

@@ -380,7 +380,7 @@ char *getCurveString(char *dest, int idx)
 
   char *s = dest;
   if (idx < 0) {
-    *s++ = '!';
+    *s++ = '-';
     idx = -idx;
   }
 
@@ -429,6 +429,24 @@ char *getValueOrGVarString(char *dest, size_t len, gvar_t value, gvar_t vmin,
   formatNumberAsString(dest, len, value, flags, 0, nullptr, suffix);
   return dest;
 }
+
+char *getValueOrSrcVarString(char *dest, size_t len, gvar_t value, gvar_t vmin,
+                           gvar_t vmax, LcdFlags flags, const char *suffix,
+                           gvar_t offset, bool usePPMUnit)
+{
+  SourceNumVal v;
+  v.rawValue = value;
+  if (v.isSource) {
+    const char* s = getSourceString(v.value);
+    strncpy(dest, s, len);
+  } else {
+    v.value += offset;
+    if (usePPMUnit && g_eeGeneral.ppmunit == PPM_US)
+      v.value = v.value * 128 / 25;
+    formatNumberAsString(dest, len, v.value, flags, 0, nullptr, suffix);
+  }
+  return dest;
+}
 #endif
 
 char *getFlightModeString(char *dest, int8_t idx)
@@ -640,7 +658,7 @@ char *getSourceString(char (&destRef)[L], mixsrc_t idx)
 
   if (idx < 0) {
     idx = -idx;
-    dest[0] = '!';
+    dest[0] = '-';
     dest += 1;
     dest_len -= 1;
   }