Integrate log4tango completely

From !971 (comment 1048335296):

• Remove the toplevel log4tango folder
• Integrate tests in /tests, see !1450 (merged)
• Move code and include into /cppapi or /src if this is done after #964 (closed)
• Integrate log4tango/config into /configure and remove separate config.h.in Not required anymore.
• Replace atoi & friends with parse_as, remove .pre-commit.yaml exception