Added videoPath to subtitle data

VideoPath reflects a video linked to loaded subtitle (not really relevant in CLI mode). User will be
able to easily fetch FPS from such video + it might be saved in some subtitle formats metadata.
Getting FPS from linked files might be placed e.g. in file list context menu.

Linking of video and subtitle might be triggered by the following conditions:
- when loading subtitles. Subconvert automatically tries to match subtitle filename base with known
  video formats. Currently videoPath is saved "forever". If video is found and FPS is successfully
  loaded, an icon will appear near FPS value on file list.
- user input (to be implemented). User might link or unlink files with any video file (chosen from
  FileDialog).