Improve way of merging tasks configurations

[RomanNikitenko]

Description

We support export configurations from devfile in a launch.json and tasks.json files.
The current implementation allows to merge existed configurations from config file and configs from devfile. Configurations from devfile have higher priority than existed configs in config file.
If you already have configs in launch.json file, for example, the configs with the same label will be replaced by configs from devfile.
So, at the moment it's expected that user should edit label/name of a configuration, which he customized.
We can improve the mechanism of merging by adding prefix for configs with the same label/name, but with different content.

[RomanNikitenko]

The behavior was improved within eclipse-che/che-theia#355: so the existed configs in config file have higher priority than the configurations from devfile. User can add new configurations to config file or edit existed ones - the changes are stored to config file. We have another issue about the sync of configurations from config file with configs from devfile.
So I'm closing the issue.