Adding a more proper GUI example (features an input text line and an echo
screen).

Is there a viable way to redirect print output to the ech screen?
Fix a bug that a procedure call with no argument was not correctly
recognized. It was always treated as a variable. Now a dangling variable
is first tried as a procedure call first before try it as a variable. This
is consistent with IDL terminal's behavior (8.3).
Use journal to redirect output from print. The output is then grabbed by
MIDLE to show as echo in the GUI. Note that PRINT is required for anything
to be recorded in journal. It does not capture the output of HELP.
The echo window is now scrollable and automatically scroll up when the
screen is filled.
The demo GUI console now tracks input history and allow navigation through
ctrl-p and ctrl-n. Add more comments to the code.
Merge branch 'gui'

Conflicts:
	CHANGES.md
Move the demo GUI application to the demo folder.
Update README for the demo.