Improve const-correctness

[elfring]

💭 I suggest to add the key word “const” to the type specifiers for a few parameters in functions like the following.

• filename
  

  libcue/cd.h

  Lines 45 to 46 in 7176a1f

  	/* filename of data file */
  	void track_set_filename(Track *track, char *filename);

• value
  

  libcue/cdtext.h

  Lines 28 to 29 in 7176a1f

  	/* set CD-TEXT field to value for PTI pti */
  	void cdtext_set(int pti, char *value, Cdtext *cdtext);

Would you like to apply the advices from an article to more places in your source files?