Add pre-loading check in WintermuteDependencies.cmake
Add a suite of methods to WintermutePluginMacros

  * Update `wintermute_plugin_declare'.
   - Add more constants about building.
   - Provide a default exportation symbol.
  * Define `wintermute_plugin_target_declare'.
   - Define a shared library for building.
   - Add default Wintermute building properties.
   - Include default and added directories.
   - Target default libraries as well as its own.
  * Stub `wintermute_plugin_generate_documentation'.
  * Define `wintermute_plugin_add_include_directories'.
  * Define `wintermute_plugin_add_libraries'.
  * Change `NAME_AUTHOR' to `AUTHOR_NAME' as property.