Simplify the use of custom script workers

Currently if a custom WD script worker is used in a workflow, then the workflow cannot be easily reused on another machine. The reason is there are ".usa" files describing custom workers structure that are required to present in at least one of a few specific folders, where UGENE keeps application settings. So in order to reuse someone's workflow a user has to copy appropriate ".usa" files to one of those implicit folders. It is better to enlist a workflow file location to a set of allowed locations for ".usa" files. In this way, the user could simply copy a workflow file along with the "*.usa" ones to any directory and use it.
Also when the "Save workflow (as)" action is used the workflow file is to be stored to the specified location along with all required "*.usa" files.