MainFrm: Get the source filter from the graph immediately after opening the main file.

Changed all code that depends on the source filter to take advantage of it.
Fixes issues with the wrong source filter being used when multiple source filters are on the graph.
CONTRIBUTING.md: fix forking link.
Updated Czech translation
Spelling correction in Spanish translation.
Add some helper functions to check if a string matches an ISO 639 code or language.
Play > Subtitles menu: Improve the external subtitles naming.

Show the subtitle filename and detect the language whenever possible.