common: split ini-config in its own convenience library

The src/common/config contains code related to two kinds of unrelated
"config": the ini config, used for configuration files, and the XML
session configuration (used for loading/saving sessions, and
incidentally MI).

Split the ini config in its own convenience library, in
src/common/ini-config and keep the rest under src/common/config.

Move ini-related things out of config/session-config.{cpp,h} and into
ini-config/ini-config.{cpp,h}.

Change-Id: Ia0b2b6cdcc15198e20444aa30f1fc86c053176d9
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>