Improve navigation with private headers

Often an object-orientated implementation consists of the following file:

• class.cpp - the implementation
• class.h - the public header
• class_p.h - the private header

While navigating with "Goto definition" already works, switching from the CPP to the private header is not possible with F4. Switching back already works.

There are two possible solutions:

1. Switch roundwise from CPP to H to P_H to CPP with every F4 press
2. Use a different shortcut (like Shift+F4) to switch between CPP and P_H

The pattern for the private headers should be customizable in Tools > Options > C++ > File Naming

The feature should also be possible for C source, as the same design pattern can be used there.