#177158 - build.properties broken after settings import

https://netbeans.org/bugzilla/show_bug.cgi?id=177158

Description: When user imports settings from Tools/Options it may break 
build.properties. Then existings projects are uncompilable because paths 
to libraries are missing.

Impact: When build.properties are broken it is not corrected by restart 
and user doesn't know how to fix it.

Risk: It doesn't influence other import/export facility, just importing 
using selected properties and it is only build.properties right now.

Changeset: http://hg.netbeans.org/core-main/rev/d35f452c4fde

Reviewers: Jiri Rechtacek, Petr Dvorak

Thanks for review,
J.