app-utils - remove gnc-module boilerplate

app-utils now is an ordinery shared library

A few bits worth mentioning:
1. it's not guile-free just yet, so instead of a gnc_module_load
   your code may have to call scm_c_use_module("gnucash app-utils");
   to expose the scm side of the app-utils api. This call has been
   added to gnucash-bin.c for example
2. while lots of noise in this commit is to rename from gncmodule-app-utils
   to gnc-app-utils, I'll point out the library has also been moved from
   <libdir>/gnucash to <libdir>. This required changes in app-util's
   CMakeLists.txt file for the install side and in the top level
   CMakeLists.txt file for the build directory structure.
3. The C side link module test has been removed as linking an ordinary
   shared library should be considered well tested by the compiler devs.
   The scheme side module load test has been slightly tweaked to no longer
   try to use gnc:module-load, but instead now checks whether the app-utils
   api is properly exposed to scheme after loading it via use-modules.
4. Dropped a completely obsolete README file.

CMakeLists.txt

@@ -868,7 +868,7 @@ configure_file(
 add_custom_target(uninstall
         COMMAND ${CMAKE_COMMAND} -P ${CMAKE_CURRENT_BINARY_DIR}/cmake_uninstall.cmake)
 
-set(_MODULES gnc-core-utils gnc-engine gnc-module gnc-backend-xml-utils gnucash-guile)
+set(_MODULES gnc-core-utils gnc-engine gnc-app-utils gnc-module gnc-backend-xml-utils gnucash-guile)
 
 if (WITH_SQL)
   list(APPEND _MODULES gnc-backend-sql)