Ensure smooth print moves even with LCD enabled

lcd_update can take so much time that the block buffer gets drained if
there are only short segments. This leads to jerky printer movements for
example in circles and a bad print quality.

This change implements a simple check: Only if the block currently
executed is long enough, run lcd_update.
This also means the printer will not show actual values on the LCD nor
will it respond to buttons pressed. A option that keeps the menu
accessible is also available.
Aditionaly, slow down if a block would be so fast that adding a new
block to the buffer would take more time. In this case, the buffer would
drain until it's empty in worst case.

Marlin/planner.h

@@ -123,6 +123,10 @@ typedef struct {
   #if ENABLED(BARICUDA)
     uint32_t valve_pressure, e_to_p_pressure;
   #endif
+  
+  #if ENABLED(ENSURE_SMOOTH_MOVES)
+    uint32_t segment_time;
+  #endif
 
 } block_t;
 
@@ -366,6 +370,17 @@ class Planner {
         return NULL;
     }
 
+    #if ENABLED(ENSURE_SMOOTH_MOVES)
+      static bool long_move() {
+        if (blocks_queued()) {
+          block_t* block = &block_buffer[block_buffer_tail];
+          return (block->segment_time > (LCD_UPDATE_THRESHOLD * 1000UL));
+        }
+        else
+          return true;
+      }
+    #endif
+
     #if ENABLED(AUTOTEMP)
       static float autotemp_max;
       static float autotemp_min;