Front-end protocol debugging variable

When setting the FGLGUIDEBUG environment variable to 1, information about GUI communication will be printed to stderr, and the GUI protocol exchange will be indented for a better readability in the front-end log window.

UNIX™ (shell) example:

$ FGLGUIDEBUG=1
$ export FGLGUIDEBUG
$ fglrun myprog 2>guidbg.txt

Note that in TUI mode, displayed screens can be dumped by setting the DBSCREENDUMP or DBSCREENOUT environment variables. This can be used to take a snapshot of the current TUI screen, for debugging or testing purpose.