Improve error message when function or trigger not import in engine (p…

…roject-flogo#218)

Improve the error message when function or trigger not installed in engine

Improve error message when function or trigger not import in engine (p…

…roject-flogo#218)

Improve the error message when function or trigger not installed in engine

Property cleanup (project-flogo#202)

* cleanup property doc; use non-deprecated method for registration

* tweak example

* cleanup doc

* cleanup doc

* cleanup doc

* automatically load built-in external property resolvers

* fix cyclic dependency

new loop scope only when there is filter for assign all case.

support DateTime comparison

Fix typo

Convert *string to string.

Fix DirectRunner engine option

update go.mod; fix test support

update go.mod; fix test support