Checking subdirectories for 'Directory polling' option

[dlnorgaard]

Additional feature request related to #137 .

"Including the possibility to search subdirectories for new files would be super useful. Any data recording software will generally create a new subdirectory for each new measurement. Adding a subdirectory search would allow the user to select the base directory in the Configuration - Operation Setting -> Directory tab, but not have to change that directory each time a new measurement is begun (which is typically several times per day). I'm not sure if searching all subdirs after each spectrum would delay things too much though. The solution might be a bit more complicated - e.g. take a survey of which subdirs exist in the beginning, keep checking all of them until new files begin appearing in one of them. At that point just look in that current subdir but keep checking if new subdirs have been added beyond what is known. In case a new subdir appears with new files, switch to that one and drop the other."