Change next-file naming functions to work on filename stems.

This paves the way for writing metadata files with similar prefixes, and avoids confusing numbering of capture files with different extensions (e.g. BAD_0000.S16 and BBD_0000.S8).

firmware/application/file.hpp

@@ -57,7 +57,7 @@ private:
 	FIL f;
 };
 
-std::string next_filename_matching_pattern(const std::string& filename_pattern);
+std::string next_filename_stem_matching_pattern(const std::string& filename_stem_pattern);
 
 namespace std {
 namespace filesystem {