CppTools: Adapt C++ locator names to reality

The new names are:

* C++ Methods in Current Document -> C++ Symbols in Current Document
* C++ Classes and Methods -> C++ Classes, Enums and Functions
* C++ Methods and Functions -> C++ Functions

Further notes:

* "C++ Classes" stays the same.
* The term "Methods" was removed since "Functions" says it all (member
  functions, non-member functions). Because of this, the "m" shortcut
  for this filter does not fit so well to the name anymore.
* "C++ Symbols in..." - "Symbols" means the same as in the C++ Search
  dialog: classes, enums, functions and declarations

Change-Id: I8e5f410f2dcb1fafe83d0a1c8607f4c17eabbb8c
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>