CMake: Add CMakeSnippetProvider

This allows to define snippets for CMakeLists.txt files. They can not
be used yet:-/

Change-Id: Iad68632798ecfe04018d08d284f9b5a8b0e564ee
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cmakeprojectmanager/cmakeprojectconstants.h

@@ -51,5 +51,8 @@ const char M_CONTEXT[] = "CMakeEditor.ContextMenu";
 // Settings page
 const char CMAKE_SETTINGSPAGE_ID[] = "Z.CMake";
 
+// Snippets
+const char CMAKE_SNIPPETS_GROUP_ID[] = "CMake";
+
 } // namespace Constants
 } // namespace CMakeProjectManager