Fix Watchdog time out during long lasting DELTA moves

During long lasting moves of a delta machine neither the temperatures are maintained nor buttons checked nor ... .
The fix is to call 'manage_heater()' and 'idle()' every now and then in the loop where the move is segmented. 
The code is abot the same as in G2_G3 wher we had the same problem with too large circles lasting too long.
Fixing #7510

Marlin/src/module/motion.cpp

@@ -594,6 +594,14 @@ float soft_endstop_min[XYZ] = { X_MIN_BED, Y_MIN_BED, Z_MIN_POS },
 
     // Calculate and execute the segments
     for (uint16_t s = segments + 1; --s;) {
+
+      static millis_t next_idle_ms = millis() + 200UL;
+      thermalManager.manage_heater();  // This returns immediately if not really needed.
+      if (ELAPSED(millis(), next_idle_ms)) {
+        next_idle_ms = millis() + 200UL;
+        idle();
+      }
+
       LOOP_XYZE(i) raw[i] += segment_distance[i];
       #if ENABLED(DELTA)
         DELTA_RAW_IK(); // Delta can inline its kinematics