gdb/python: add Type.is_signed property

gdb/python: add Type.is_signed property

Add a new read-only property, Type.is_signed, which is True for signed
types, and False otherwise.

This property should only be read on types for which Type.is_scalar is
true, attempting to read this property for non-scalar types will raise
a ValueError.

I chose 'is_signed' rather than 'is_unsigned' in order to match the
existing Architecture.integer_type method, which takes a 'signed'
parameter.  As far as I could find, that was the only existing
signed/unsigned selector in the Python API, so it seemed reasonable to
stay consistent.