File browser

Implements:

• File browser instead of absolute path input fields in the FilePathInput prefab (using https://assetstore.unity.com/packages/tools/gui/runtime-file-browser-113006);
• Standardization of path management using the class Path of System.IO.

Fixes:

• View mask staying active when we stop an experiment during a break;
• Eye-tracking data export filesstill created even though the HMD is missing.