gtk: Fix transfer annotations gtk_binding_set_new and *_by_class

Those instances are exclusively handled internally and can not be freed.
Mark the GdkColor struct as deprecated
tests textbuffer: don't use GdkColor (deprecated)
GtkTextView: various code clean-ups

- only one blank line is enough to separate code sections.
- the 'signals' variable was in the middle of function prototypes.
- compare pointers to NULL in some conditions ("if(blah) should be used
  only if blah is a boolean variable). It makes the code clearer.
- various other things.
Unit tests for the empty last line

For functions using _gtk_text_buffer_get_line_log_attrs():
- gtk_text_buffer_backspace()