gdb/python: new function to add values into GDB's history

gdb/python: new function to add values into GDB's history

The guile API has (history-append! <value>) to add values into GDB's
history list.  There is currently no equivalent in the Python API.

This commit adds gdb.add_history(<value>) to the Python API, this
function takes <value> a gdb.Value (or anything that can be passed to
the constructor of gdb.Value), and adds the value it represents to
GDB's history list.  The index of the newly added value is returned.