pcbnew: align centers to the top and left

When aligning module centers, it can be hard to determine which item
will be chosen for the alignment target when it chooses based on the
median of the center values.  Instead, this patch chooses the alignment
target as the top and left most items of the Y and X centering,
respectively

pcbnew/tools/placement_tool.cpp

@@ -457,8 +457,9 @@ int ALIGN_DISTRIBUTE_TOOL::AlignCenterX( const TOOL_EVENT& aEvent )
     BOARD_COMMIT commit( frame );
     commit.StageItems( selection, CHT_MODIFY );
 
-    // after sorting use the x coordinate of the middle item as a target for all other items
-    const int targetX = itemsToAlign.at( itemsToAlign.size() / 2 ).second.GetCenter().x;
+    // after sorting use the center x coordinate of the leftmost item as a target
+    // for all other items
+    const int targetX = itemsToAlign.begin()->second.GetCenter().x;
 
     // Move the selected items
     for( auto& i : itemsToAlign )
@@ -497,8 +498,9 @@ int ALIGN_DISTRIBUTE_TOOL::AlignCenterY( const TOOL_EVENT& aEvent )
     BOARD_COMMIT commit( frame );
     commit.StageItems( selection, CHT_MODIFY );
 
-    // after sorting use the y coordinate of the middle item as a target for all other items
-    const int targetY = itemsToAlign.at( itemsToAlign.size() / 2 ).second.GetCenter().y;
+    // after sorting use the center y coordinate of the top-most item as a target
+    // for all other items
+    const int targetY = itemsToAlign.begin()->second.GetCenter().y;
 
     // Move the selected items
     for( auto& i : itemsToAlign )