Multithreading support in filter

Filter module rewritten to multithreaded (filter version changed to 4); as a result, when running with no-GIL Python versions, speed is proportional to physical CPU core number; e.g., on my laptop filtering speed is doubled as compared to "classic" version 3.

"Averager" GUI changed from single window to modular, providing more efficient screen usage for large images.

"POV-Ray Thread" GUI remains single-window since "POV-Ray Thread" is not supposed to be used for large source images.