To implement AgsEditor specified XML file IO

Advanced Gtk+ Sequencer has a need for enhanced get/set abilities in regard of first stable pre release 0.4.0. The modularity of ags is one of its main goals.

Further there should be a fix for high resolution buffers. ags_devout.c and ags_audio_loop.c need to be fixed.

It follows the wanted changes to ags_file_gui.c as DTD:

<!ELEMENT ags-editor (ags-toolbar, ags-editor-pane, ags-notebook)>
<!ATTLIST ags-editor
          id               ID         #REQUIRED
          version          CDATA      #REQUIRED
          build-id         CDATA      #REQUIRED
          flags            PCDATA     #REQUIRED
          >
<!ELEMENT ags-editor-pane (IDREF*)>
<!ATTLIST ags-editor-pane
          id               ID         #REQUIRED
          >
<!ELEMENT ags-toolbar EMPTY>
<!ATTLIST ags-toolbar
          id               ID         #REQUIRED
          edit-mode        CDATA      "edit"
          tic              CDATA      "1/1"
          zoom             CDATA      "1:4"
          mode             CDATA      "multi-channel"
          >
<!ELEMENT ags-notebook (ags-notebook-tab-list)>
<!ATTLIST ags-notebook
          id               ID         #REQUIRED
          >
<!ELEMENT ags-notebook-tab-list (ags-notebook-tab*)>
<!ATTLIST ags-notebook-tab-list
          id               ID         #REQUIRED
          >
<!ELEMENT ags-notebook-tab EMPTY>
<!ATTLIST ags-notebook-tab
          id               ID         #REQUIRED
          notation         IDREF      #IMPLIED
          >