Finishing header split

[staxfur]

This are some things that still need to be done to finish the header spliting:

• Implement Term::Private::read_raw() second functionality again if no keyboard was initialized
• make Term::Private::BaseTerminal a function
• remove platform header from the base.hpp header
• remove obsolete comments
• remove undefinition from the prompt.hpp header

[staxfur]

Working on the BaseTerminal class is not needed anymore as the fix in #165 removed the headers that caused those problems.