Add comment box to Sugar Journal detail view

This series of patches adds support for a comment field to the Sugar Journal detail view. The relevant feature request is [1].

Patch 0001 modifies scrolled_window to accept multiple widget types

Patch 0002 makes a method from the write_entry code so that it can be reused

Patch 0003 adds the comment widget

Patch 0004 adds the comment box

[1] http://wiki.sugarlabs.org/go/Features/Comment_box_in_journal_detail_view