Reconcile style of include directives across all source files.

CODING-STYLE.md

@@ -1,6 +1,10 @@
 Glabels Coding Style
 ====================
 
+This file describes the coding style used in all glabels source code.  Any patches or pull requests should
+adhere to this style.
+
+
 Tabs vs. Spaces
 ---------------
 
@@ -40,3 +44,53 @@ else
 Also applies to class and namespace declaration statements.
 
 See https://en.wikipedia.org/wiki/Indent_style#Allman_style for more information.
+
+
+File Organization
+-----------------
+
+Generally code is organized into modules.  Usually a module defines a single class or a small namespace of
+functions/constants/etc.  A module is defined by two files: a header file (its specification) and an
+implementation file.  Header filenames have a ".h" extension and implementation filenames have a ".cpp"
+extension.
+
+
+### Self-contained Headers
+
+Header files should be self-contained.  I.e. they should not require any prerequisite includes.  To enforce
+this requirement, an implementation file shall include its header file before any other includes.
+
+
+### Multiple Inclusion Guards
+
+All header files should have an ifndef guard to prevent multiple inclusion.
+
+```
+#ifndef ns_Module_h
+#define ns_Module_h
+
+...
+
+#endif // ns_Module_h
+```
+
+### Include Directives
+
+Header files should be included in the following order.
+
+1. header file for this module (e.g. this would be "Foo.h" in "Foo.cpp").
+2. C system header files (preference is for the C++ version if available, e.g. <cmath> instead of <math.h>.
+3. C++ system header files (e.g. STL files)
+4. Qt header files
+5. Other libraries' header files
+6. Other glabels header files.
+
+Paths used in include directives should always be relative to either the glabels source directory or an
+appropriate base directory for each library.  They should NEVER include UNIX directory shortcuts such as "."
+(the current directory) or ".." (the parent directory).
+
+Angle brackets ("<>") should be used for inclusion of all external header files (such as C/C++ and Qt
+header files). Double quotes should be used for all glabels header files.
+
+Do not use forward declarations to any external entities.  Use the appropriate include directive instead.
+