Support customization of configuration area for customer application

[rschnekenbu]

What it does

Adds an application prop to define the location of the configuration area.
This property can be used by theia-based applications to customize their own configuration area. The logic of the THEIA_CONFIG_DIR environment variable should still works and it should override this application prop, without the need of overriding the environment variable server.

fixes #14318

Contributed on behalf of STMicroelectronics

How to test

• Sample application can be used without modification. '.theia' should be the default configuration folder (default prop is used)
• Setting an env variable to another folder (export THEIA_CONFIG_DIR=".theia-configDir") for example, and re-test with the sample application.
• Unset the environment variable and setting the application property in the sample application package.json to "configurationFolder" : ".theia-example". Config dir shall be $home/.theia-example
• Set again the env property. The environment property should override the configurationFolder value

Follow-ups

• Update theia-blueprint to use this application prop rather than then env variable server mechanism. (see Remove workaround for configuration space  theia-ide#393)

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines