Upgrade to C++17 [CMake] (#4717)

* Make our CMake slightly less insane

CMake/GroupSources.cmake

@@ -0,0 +1,42 @@
+# Enable the support for solution folders in MSVC
+set_property(GLOBAL PROPERTY USE_FOLDERS ON)
+
+# Put projects into solution folders in MSVC:
+set_target_properties(
+	event_core_static
+	event_extra_static
+	expat
+	fmt
+	jsoncpp_lib
+	lua
+	luaexpat
+	mbedcrypto
+	mbedtls
+	mbedx509
+	lsqlite
+	sqlite3
+	SQLiteCpp
+	tolualib
+	zlib
+	PROPERTIES FOLDER Libraries
+)
+
+# luaproxy not generated on anything else
+if(WIN32)
+	set_target_properties(
+		luaproxy
+		PROPERTIES FOLDER Support
+	)
+endif()
+
+if(${BUILD_TOOLS})
+	set_target_properties(
+		MCADefrag
+		ProtoProxy
+		PROPERTIES FOLDER Tools
+	)
+endif()
+
+# Put all files into one project, separate by the folders:
+get_property(TARGET_SOURCE_FILES TARGET ${CMAKE_PROJECT_NAME} PROPERTY SOURCES)
+source_group(TREE "${PROJECT_SOURCE_DIR}/src" FILES ${TARGET_SOURCE_FILES})