[DOC] Update README.md.

[FIX] Remove deprecation warning.

- For string concatenation.

[DEV] Replace set/clear/getAccessor() with accessor() properties.

[DOC] Use accessor() property, and document it.

[DOC] Ready for 2.5.0 doc generation.

[DOC] Update documentation for EntitySysD 2.5.0

[DOC] Update documentation for EntitySysD 2.4.0

[BUILD] Upgrade example dependencies.

[DOC] Document various new stuff.

- Also add comment from rcorre in its fork (commit 17295f).

[TEST] Test subscribing an already subscribed event.

- It should ignore it silently.