Eeschema: fix large diffs when saving schematics.

Apparently the < operator was never implemented for SCH_JUNCTION objects
so they were not get sorted which was causing the large diffs between
schematic saves.

Fixes https://gitlab.com/kicad/code/kicad/issues/4369

eeschema/sch_junction.cpp

@@ -203,3 +203,22 @@ BITMAP_DEF SCH_JUNCTION::GetMenuImage() const
 {
     return add_junction_xpm;
 }
+
+
+bool SCH_JUNCTION::operator <( const SCH_ITEM& aItem ) const
+{
+    if( Type() != aItem.Type() )
+        return Type() < aItem.Type();
+
+    if( GetLayer() != aItem.GetLayer() )
+        return GetLayer() < aItem.GetLayer();
+
+    auto junction = static_cast<const SCH_JUNCTION*>( &aItem );
+
+    if( GetPosition().x != junction->GetPosition().x )
+        return GetPosition().x < junction->GetPosition().x;
+
+    return GetPosition().y < junction->GetPosition().y;
+
+}
+

eeschema/sch_junction.h

@@ -104,6 +104,8 @@ public:
 
     EDA_ITEM* Clone() const override;
 
+    virtual bool operator <( const SCH_ITEM& aItem ) const override;
+
 #if defined(DEBUG)
     void Show( int nestLevel, std::ostream& os ) const override;
 #endif