ENH: qvtkConnect and qvtkDisconnect performance improvement

Whenever a new connection is added or deleted, it's searched in the list of already existing connections using ctkVTKObjectEventsObserverPrivate::findConnection(). This findConnection() method is very slow if there are many connections, because the time-consuming ctkVTKConnection::isEqual() method is executed for each existing connection (before this commit, 40% of the time was spent in vtkConnection::isEqual while creating 10000 connections).

To solve this performance issue, added a ConnectionIndex structure, which stores a pointer to each connection object using a hash map. When searching for a fully specified connection, the ConnectionIndex is used to find the connection directly, instead of iterating through all the children.

Result of the optimization (ctkVTKObjectTest1):
Create 10000 connections: 2.343 sec => 1.019 sec (2.3x speed improvement)
Remove 10000 connections: 3.599 sec => 2.031 sec (1.77x speed improvement)