Split new GeanyEditor struct type from GeanyDocument fields.
Add GeanyDocument::editor field.
GeanyEditor::document allows access back to document fields.
GeanyEditor::scintilla replaces GeanyDocument::sci.


git-svn-id: https://geany.svn.sourceforge.net/svnroot/geany/branches/editor-struct@2763 ea778897-0a13-0410-b9d1-a72fbfd435f5