Enhance rebar integration

[deadok22]

• Rebar project can contain multiple applications, which are represented as separate modules in an IntelliJ project, and these projects can be configured with a single rebar.config file, which means we need to add some project-level builder for such configurations.
• Rebar configuration files in subdirectories are to be considered: properties set in root rebar.config file are overriden by those contained in nested directories' rebar.config. It affects both build process and include path settings during project import.

This issue has it's root in #405