manager: Allow to add plugin dir via environent

This eases testing of plugins from the source tree:

  CALLS_PLUGIN_DIR=_build/plugins/dummy/ _build/src/gnome-calls -p dummy

src/calls-manager.c

@@ -86,12 +86,19 @@ load_provider (const gchar* name)
   PeasEngine *plugins;
   PeasPluginInfo *info;
   PeasExtension *extension;
+  const gchar *dir;
 
   // Add Calls search path and rescan
   plugins = peas_engine_get_default ();
   peas_engine_add_search_path (plugins, PLUGIN_LIBDIR, NULL);
   g_debug ("Scanning for plugins in `%s'", PLUGIN_LIBDIR);
 
+  dir = g_getenv ("CALLS_PLUGIN_DIR");
+  if (dir && dir[0] != '\0') {
+    g_debug ("Adding %s to plugin search path", dir);
+    peas_engine_prepend_search_path (plugins, dir, NULL);
+  }
+
   // Find the plugin
   info = peas_engine_get_plugin_info (plugins, name);
   if (!info)