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.

backends/merge/CMakeLists.txt

@@ -20,7 +20,7 @@ set (merge_sources
 )
 
 set (merge_qobject_headers
-  Merge.h
+  Merge.hpp
 )
 
 qt6_wrap_cpp (merge_moc_sources ${merge_qobject_headers})
@@ -33,10 +33,9 @@ add_library (Merge STATIC
   ${merge_moc_sources}
 )
 
-#target_compile_features (Merge
-#  PUBLIC cxx_std_11
-#)
-set_property (TARGET Merge PROPERTY CXX_STANDARD 11)
+target_compile_features (Merge
+  PUBLIC cxx_std_20
+)
 
 target_include_directories (Merge
   PUBLIC ..