Merge keypress.cpp & input.cpp

Modification according to #6255, hope I'm moving in the correct direction.

Details:
- Merge function 'convert_to_dialog_key' with function 'input'.
- Delete unused function 'input_wait'.
- Move functions 'input' and 'get_keypress' to input.cpp.
- Delete files keypress.h & keypress.cpp.
- Update project files.