Big-Ugly Style Update (#278)

* Bulk replaced tabs with spaces
* Bulk removed trailing whitespace from lines
* Replaced c-style comments with c++-style comments in file banners
* Replace nested namespace definitions with single concise definitions (C++17), this keeps the indentation more manageable
* Cleanup ordering and spacing of include directives
* Bulk renaming of header file extensions from '.h' to '.hpp'.
* Update CODING-STYLE.md
* Update target_compile_features from cxx_std_11 to cxx_std_20.
* Refresh .clang-format file.  Still needs a lot of tweaking.

docs/CODING-STYLE.md

@@ -4,29 +4,20 @@ 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.
 
-:bulb: **Note:** The top-level glabels source directory includes a *.clang-format* file.
-This file can be used with `clang-format` to reformat code to a reasonable approximation
-of the style described here.
-
 
 Formatting
 ----------
 
 ### Tabs vs. Spaces
 
-Tabs are only used at the beginning of a line, and only used to express
-indentation level.  Spaces are used for any other type of vertical alignment,
-e.g. aligning function arguments.  This ensures that the code displays
-correctly everywhere, regardless of the viewer's tab size, and does not inflict
-the viewer with my choice of tab size (8 spaces).
-
-I use the emacs smart-tabs-mode to automatically enforce this.  See
-https://www.emacswiki.org/emacs/SmartTabs for more information.
+Use spaces for all code indentation and line-to-line alignment of code elements.
+(e.g. aligning function arguments).  This ensures that the code displays
+correctly everywhere.
 
 
 ### Indentation Style
 
-Glabels code uses the Allman style (a.k.a "BSD Style") of code indentation.
+Glabels code uses the Allman[^1] style (a.k.a "BSD Style") of code indentation.
 I.e. the brace associated with a control statement is placed on the next line,
 indented to the same level as the control statement.  Statements within the
 braces are indented to the next level.
@@ -50,8 +41,6 @@ else
 
 Also applies to function, class and namespace declaration statements.
 
-See https://en.wikipedia.org/wiki/Indent_style#Allman_style for more
-information.
 
 
 ### Parenthesis
@@ -98,8 +87,8 @@ Code 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.
+Header filenames have a `.hpp` extension[^2] and implementation filenames have a
+`.cpp` extension.
 
 
 ### Self-contained Headers
@@ -114,8 +103,8 @@ shall include its header file before any other includes.
 All header files should have an ifndef guard to prevent multiple inclusion.
 
 ```
-#ifndef ns_Module_h
-#define ns_Module_h
+#ifndef ns_Module_hpp
+#define ns_Module_hpp
 
 ...
 
@@ -127,7 +116,7 @@ All header files should have an ifndef guard to prevent multiple inclusion.
 
 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").
+1. header file for this module (e.g. this would be `Foo.hpp` in `Foo.cpp`).
 2. Glabels header files.
 3. Qt header files
 3. C++ system header files (e.g. STL files)
@@ -152,3 +141,33 @@ include directives instead.
 
 - Private definitions are placed in unnamed namespaces to limit scope to the current translation unit.
 - All other glabels code is placed in the top-level "glabels" namespace.
+- Nested namespace declarations should follow the more concise C++17 syntax
+```
+namespace glabels::model
+{
+        ...
+}
+```
+
+
+Notes
+-----
+
+### Emacs
+
+The following is an excerpt from my emacs configuration supporting the style
+```
+(defun my-c++-hook ()
+    (setq c-default-style "bsd"
+          c-basic-offset 8
+          indent-tabs-mode nil
+          show-trailing-whitespace t))
+
+(add-hook 'c++-mode-hook 'my-c++-hook)
+```
+
+
+References
+----------
+[^1]: https://en.wikipedia.org/wiki/Indent_style#Allman_style
+[^2]: [C++ Weekly - Ep 442 - Stop Using .h For C++ Header Files!](https://www.youtube.com/watch?v=mr3sOT-Delg)