Clean up co-linear tracks after finishing routing

This involved adding some extra infrastructure to be able
to handle the case where a track that is sitting in the
router's commit waiting to be added to the board actually
needs to be deleted instead.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/8419

include/commit.h

@@ -130,6 +130,12 @@ public:
     virtual COMMIT& Stage( const PICKED_ITEMS_LIST& aItems,
                            UNDO_REDO aModFlag = UNDO_REDO::UNSPECIFIED );
 
+    /**
+     * Removes an item that was previously staged to this commit from the commit.
+     * Use this with care:
+     */
+    virtual COMMIT& Unstage( EDA_ITEM* aItem );
+
     ///< Execute the changes.
     virtual void Push( const wxString& aMessage = wxT( "A commit" ),
                        bool aCreateUndoEntry = true, bool aSetDirtyBit = true ) = 0;
@@ -145,6 +151,11 @@ public:
     ///< Returns status of an item.
     int GetStatus( EDA_ITEM* aItem );
 
+    /**
+     * @return a list of items added by this commit
+     */
+    std::vector<EDA_ITEM*> GetAddedItems() const;
+
 protected:
     struct COMMIT_LINE
     {