Overload the assignment operators for the Table and Index classes

This fixes issues that assigning the main representation of the database
in DBBrowserDB::objectMap is modified when playing around with the
database in the Edit Table and Edit Index dialogs.

These bugs were caused by my refactoring. Let's hope there's not much
more fallout from this refactoring...

src/sqlitetypes.h

@@ -238,6 +238,7 @@ class Table : public Object
 public:
     explicit Table(const QString& name): Object(name), m_rowidColumn("_rowid_") {}
     virtual ~Table();
+    Table& operator=(const Table& rhs);
 
     virtual Types type() const { return Object::Table; }
 
@@ -337,6 +338,7 @@ class Index : public Object
 public:
     explicit Index(const QString& name): Object(name), m_unique(false) {}
     virtual ~Index();
+    Index& operator=(const Index& rhs);
 
     virtual Types type() const { return Object::Index; }