Add new CONNECTIVITY_CANDIDATE flag

We reuse the CANDIDATE flag in various places across the codebase,
which is probably incorrect given the original intention for the
flag. Clearing it before use would be a performance hit for incremental
connectivity - and we can't rely on other users of the flag to clear
it for us. So we now use a new flag just for connectivity to avoid
other users trampling on connectivity updates.

eeschema/connection_graph.cpp

@@ -1450,10 +1450,10 @@ void CONNECTION_GRAPH::buildItemSubGraphs()
                         [&]( SCH_ITEM* aItem ) -> bool
                         {
                             SCH_CONNECTION* conn = aItem->GetOrInitConnection( sheet, this );
-                            bool unique = !( aItem->GetFlags() & CANDIDATE );
+                            bool            unique = !( aItem->GetFlags() & CONNECTIVITY_CANDIDATE );
 
                             if( conn && !conn->SubgraphCode() )
-                                aItem->SetFlags( CANDIDATE );
+                                aItem->SetFlags( CONNECTIVITY_CANDIDATE );
 
                             return ( unique && conn && ( conn->SubgraphCode() == 0 ) );
                         };
@@ -1480,7 +1480,7 @@ void CONNECTION_GRAPH::buildItemSubGraphs()
 
                         for( SCH_ITEM* citem : citemset )
                         {
-                            if( citem->HasFlag( CANDIDATE ) )
+                            if( citem->HasFlag( CONNECTIVITY_CANDIDATE ) )
                                 continue;
 
                             if( get_items( citem ) )
@@ -1490,7 +1490,7 @@ void CONNECTION_GRAPH::buildItemSubGraphs()
                 }
 
                 for( SCH_ITEM* connected_item : memberlist )
-                    connected_item->ClearFlags( CANDIDATE );
+                    connected_item->ClearFlags( CONNECTIVITY_CANDIDATE );
 
                 subgraph->m_dirty = true;
                 m_subgraphs.push_back( subgraph );

include/eda_item.h

@@ -156,7 +156,8 @@ public:
 
     EDA_ITEM_FLAGS GetTempFlags() const
     {
-        constexpr int mask = ( CANDIDATE | SELECTED_BY_DRAG | IS_LINKED | SKIP_STRUCT | SELECTION_CANDIDATE );
+        constexpr int mask = ( CANDIDATE | SELECTED_BY_DRAG | IS_LINKED | SKIP_STRUCT | SELECTION_CANDIDATE
+                               | CONNECTIVITY_CANDIDATE );
 
         return m_flags & mask;
     }

include/eda_item_flags.h

@@ -60,7 +60,7 @@
 
 #define ROUTER_TRANSIENT      (1 << 22)     ///< transient items that should NOT be cached
 
-// 23 is unused
+#define CONNECTIVITY_CANDIDATE ( 1 << 23 ) ///< flag indicating that the structure is connected for connectivity
 
 #define HOLE_PROXY     (1 << 24)   ///< Indicates the BOARD_ITEM is a proxy for its hole
 #define SHOW_ELEC_TYPE (1 << 25)   ///< Show pin electrical type