excluding idea project files from git
Eliminating useless TaskBuilder.setTraits() method
Making TaskBuilderImpl.setValidatable() method package private.
Using diamond syntax where applicable.
Removed all the javadoc - we don't put it on impl. classes.
Implemented support for child tasks installed from Revertible component.
Such child tasks can only have Executable and Validatable components.
Revertible components are forbiden to prevent endless transactions.
There's ugly hack in place (documented for now) to make this feature work.