Web UI: Manually added properties in property bag are lost in app import/export

[vijaynalawade]

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[] feature request
[] support request
[] general question

Current behavior (how does the issue manifest):
Manually added properties are removed upon export of app. I had to manually add them again.

Expected behavior:
Properties must be preserved upon import/export.

Minimal steps to reproduce the problem (not required if feature enhancement):

1. Export app
2. Add few properties manually as described at https://tibcosoftware.github.io/flogo/development/flows/property-bag/
3. Import app back into Web UI
4. Export same app again

What is the motivation / use case for changing the behavior?
It is very inconvenient to add them again and again on every export.

Please tell us about your environment (Operating system, docker version, browser & verison if webui, etc):
0.5.2
Flogo version (CLI & contrib/lib. If unknown, leave empty or state unknown): 0.X.X

Additional information you deem important (e.g. issue happens only occasionally):