Finish release 1.4.0

1) Improve and clarify documentation:
 - update, reformat, and reorder parts of CREDITS.txt
 - make sure that documentation appears in doxygen docs
 - add '\since 1.4.0' to documentation of new methods and functions
 - remove doxygen's '\brief' where not required (we're using
   JAVADOC_AUTOBRIEF = YES)
 - clarify screen scaling, scaling factor, and related stuff
- add more info about backwards compatibility for X11 specific code

2) Update dependencies