Remove $Id$ tags, update URL's, and more

- remove obsolete svn '$Id$' tags from all source files
- update .fl files and generated files accordingly
- replace 'http://www.fltk.org' URL's with 'https://...'
- replace bug report URL 'str.php' with 'bugs.php'
- remove trailing whitespace
- fix other whitespace errors flagged by Git
- add and/or fix missing or wrong standard headers
- convert tabs to spaces in all source files

The only relevant code changes are in the fluid/ folder where
some .fl files and other source files were used to generate
the '$Id' headers and footers.

src/fl_overlay.cxx

@@ -1,6 +1,4 @@
 //
-// "$Id$"
-//
 // Overlay support for the Fast Light Tool Kit (FLTK).
 //
 // Copyright 1998-2018 by Bill Spitzak and others.
@@ -9,11 +7,11 @@
 // the file "COPYING" which should have been included with this file.  If this
 // file is missing or damaged, see the license at:
 //
-//     http://www.fltk.org/COPYING.php
+//     https://www.fltk.org/COPYING.php
 //
-// Please report all bugs and problems on the following page:
+// Please see the following page on how to report bugs and issues:
 //
-//     http://www.fltk.org/str.php
+//     https://www.fltk.org/bugs.php
 //
 
 // Extremely limited "overlay" support.  You can use this to drag out
@@ -133,7 +131,3 @@ void fl_overlay_rect(int x, int y, int w, int h) {
   px = x; py = y; pw = w; ph = h;
   draw_current_rect();
 }
-
-//
-// End of "$Id$".
-//