Hidden blocks only in the Placement UI (#5806)

* placeablePlayer

* placeablePlayer unlocked

core/src/mindustry/ui/fragments/PlacementFragment.java

@@ -443,7 +443,7 @@ public class PlacementFragment extends Fragment{
     }
 
     boolean unlocked(Block block){
-        return block.unlockedNow();
+        return block.unlockedNow() && block.placeablePlayer;
     }
 
     boolean hasInfoBox(){

core/src/mindustry/world/Block.java

@@ -100,6 +100,8 @@ public class Block extends UnlockableContent{
     public boolean requiresWater = false;
     /** whether this block can be placed on any liquids, anywhere */
     public boolean placeableLiquid = false;
+    /** whether this block can be placed directly by the player via PlacementFragment */
+    public boolean placeablePlayer = true;
     /** whether this floor can be placed on. */
     public boolean placeableOn = true;
     /** whether this block has insulating properties. */