[GTK] Properly convert single file names to tuple
[GTK] Properly convert single file names to tuple

If file name is a single string, tuple(string) splits it into individual
characters.

Also Unicode encode the strings to match behaviour on other platforms.

r0x0r/pywebview,shivaprsdv/pywebview,r0x0r/pywebview,r0x0r/pywebview,r0x0r/pywebview,r0x0r/pywebview,shivaprsdv/pywebview,shivaprsdv/pywebview,shivaprsdv/pywebview