Tweak appearance of rows & expanders

Investigate enhancements to appearance of rows & their expanders.

It may be possible to create aligned expanders and bullet points by using 2x custom pixbuf renderer. If possible, it would look very similar to an infamous Mac OS application.

1. Set first renderer to be expander columns
2. Create bullet point pixbuf in second column
3. Make both columns equal in size of pixbuf images
4. For row with children, show col1 and hide col2
5. For row with no children, hide col1 and show col2

If this looks better, then it should be implemented.