update/refactoring AgsEditor in front of 0.4.0

ags-buffer recall implemented in recent subversion tree. Additionaly extended AgsFFPlayer to play notation.

AgsEditor is not grained. Therefore an important update to make it more gained is comming soon.

• move GtkDrawingArea of AgsEditor to AgsNoteEdit
• implement saving editor related settings

additional goals

• test AgsPlayNotationAudioRun