Create class for finding D-Bus interfaces

Create a class for finding instances of D-Bus interfaces.  The class
finds existing instances using the ObjectMapper.  It also finds new
instances using an InterfacesAdded listener.

Tested:
* See test plan at
  https://gist.github.com/smccarney/67d7719f0cf9cea2d134b0796aa408da

Change-Id: Ia6fba9cb69ad73607dc971dd6d63ba2efecabe0d
Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>
diff --git a/meson.build b/meson.build
index 7d5dd11..bc0f16f 100644
--- a/meson.build
+++ b/meson.build
@@ -154,6 +154,7 @@
     'power',
     error_cpp,
     error_hpp,
+    'dbus_interfaces_finder.cpp',
     'gpio.cpp',
     'pmbus.cpp',
     'temporary_file.cpp',