Move provider plugins into a dedicated directory

Since we will introduce another type of plugin for the policy engine
we want to have each plugin type in separate directories.

We also have to adjust:

- plugin search directories
- po file location
- update paths for calls-doc target

plugins/provider/ofono/libgdbofono/manager.xml

@@ -0,0 +1,13 @@
+<!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
+"http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
+<node>
+
+  <interface name="org.ofono.Manager"><method name="GetModems"><arg name="modems" type="a(oa{sv})" direction="out"/>
+</method><signal name="ModemAdded"><arg name="path" type="o"/>
+<arg name="properties" type="a{sv}"/>
+</signal>
+<signal name="ModemRemoved"><arg name="path" type="o"/>
+</signal>
+  </interface>
+
+</node>