feature CtScannerListener listens for nodes of EarlyTerminatingScanner

[pvojtechovsky]

As discussed in #1180 there is an new contract of EarlyTerminatingScanner. There is possible to register implementation of CtScannerListener, which can be used to listen for entering/exiting of node scanning and which can influence, whether node and child nodes are scanned, by return value of CtScannerListener#enter

There is no test. It will be indirectly tested by future PRs.

[monperrus]

Excellent, baby steps are great!

Is it possible to have better typing such as

enter(CtElement element); // instead of Object
exit(CtElement element);

[pvojtechovsky]

Good idea, I came to some one at the same time too. ;-)

Done.

[monperrus]

Just made a doc PR on this one: pvojtechovsky#13