Use gimp_input_data_stream_read_line_always(), instead of
g_input_data_stream_read_line(), in a bunch of places that don't
expect EOF.  If we don't do that, the code assumes the GError
parameter is set by the function and returns an error indication,
causing the caller to segfault when it tries to access
error->message.  Instead, we now process an empty line when EOF is
reached, which is caught by the normal parsing logic.

Additionally:

  - Use gimp_ascii_strto[id]() when loading gradients, generated
    brushes, and palettes, to improve error checking for invalid
    numeric input.

  - Improve gradient-segment endpoint consistency check.

  - Allow loading palette files with 0 colors.  They can be created
    during the session, so we might as well successfully load them.