Fixed GUI spawning

* Added SubFile instead of Utils module. It stores a file infoo and is a wrapper to reading and
  writing to the filesystem.
* GUI now uses new 5-step mode of work: open-parse-edit-convert-write. Currently partial opening and
  parsing are implemented.
* Parsing results are stored inside a fileStorage (dictionary containing pair of file info and
  parsed content). TODO: They should be removed on file close.
* Fixed format extensions in file open dialog
* Some other bugs are fixed.