Added public API to execute Gradle commands (NbGradleProject does not yet implement it).
Fixed spelling errors in javadocs.
Replaced TaskCompleteListener with CommandCompleteListener.
Refactored asynchronous task execution.
GradleCommandExecutor is now available on the project's lookup.
Fixed a possible NPE.
Fixed an NPE in StandardTaskVariable.
Fixed a type in the javadoc.
Minor refactoring in GradleProjectLogicalViewProvider.