Refactor LabelFileFormat object and create abstract class for Reader and Writer objects

[ZachKLYeh]

Refactor of LabelImg application:

1. LabelFileFormat: Create a class for LabelFileFormat instead of enum. All file formats should follow the class by specifying extension, reader and writer objects. The file extensions are saved in classes and the detection of file type can be done by loop iterations.
2. Abstract class for Reader and Writer: Allignment of IO operation for all reader and writers, this makes loading and saving file a static operation, rather than different function by the original design.