inherit class UnoTreeListItem from SvLBoxString instead of SvLBoxItem so that
it really isa SvLBoxString and can be edited with startEditingAtNode

a) remove the unused SvLBoxEntry argument to SvLBoxString's SetText
b) used rtl::OUString instead of XubString
c) rename aStr to maText
d) inherit UnoTreeListItem from SvLBoxString and drop its maText in
   favour of that of SvLBoxEntry

Change-Id: Id78f43ae5be8cd6f581e6c2fd2bd6f46de9144ba