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/sip/sip.gresources.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<gresources>
+  <gresource prefix="/org/gnome/Calls/ui">
+    <file preprocess="xml-stripblanks">sip-account-widget.ui</file>
+  </gresource>
+</gresources>