Fix to cater for changing API in anjuta version 3.6.2+

* Contribute to future proofing against new versions of anjuta library
Fixes to cater for changing API in anjuta version 3.8.0+

* Contribute to future proofing against new versions of anjuta library

* core_prefs.c
* coverweb.c
* plugin.c
* anjuta-app.c
 * Fixes error messages about destroyed widgets being already destroyed

* gtkpod.c
* anjuta-about.c
 * Makes changes to align code more closely with the anjuta 3.8.0 codebase