Change double-click behaviour on account hierarchy

- placeholder accounts will expand/collapse
- all other account types will open the register

This is a partial fix for bug 721026 and will
alleviate the biggest frustration experienced there

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23752 57a11ea4-9604-0410-9ed3-97b8803252fd