Allow definition of configuration folder in the application properties

[rschnekenbu]

Despite #13708, it is currently impossible to use the THEIA_CONFIG_DIR environment variable in a custom theia application.

In fact, the advised method for theia applications is to override the environment variable server and the getConfigDir() method. (see #4488 and https://github.com/eclipse-theia/theia/blob/master/doc/changelogs/CHANGELOG-2020.md#v0160---27022020)
In this case, all implemented logic about THEIA_CONFIG_DIR is ignored, as the config directory returned is always the one defined in the custom environment variable server.

Configuration folder could be set in the application props, so that any custom application could set its own folder. Default value for this one should be obviously '.theia' for compatibility reasons.