[Libreoffice] Fix an old bug 78701

This bug is almost 4-years old - 
http://openoffice.org/bugzilla/show_bug.cgi?id=78701

Symptoms:
  calling startEditingAtNode(nodeobject) method of a tree control does 
nothing

What should happen:
  Tree item editing should start. Depending on the implementation it 
might look like a text control floating over the item, or the item 
itself will turn into something editable; i don't know, since, 
apparently, no one have ever been able to put a node in this mode.

What actually happens:
  Nothing happens. isEditing() returns False after startEditingAtNode().

Why it happens:
  As explained in the original bug report, this code ( void 
SvTreeListBox::ImplEditEntry( SvLBoxEntry* pEntry ) in svtreebx.cxx):
  ...
          for( sal_uInt16 i = 0 ; i < nCount ; i++ )
          {
              SvLBoxItem* pTmpItem = pEntry->GetItem( i );
              if( pTmpItem->IsA() != SV_ITEM_ID_LBOXSTRING )
                  continue;
  ...
  checks that a tree item is a SV_ITEM_ID_LBOXSTRING and refuses to do 
anything with it otherwise.
  However, this code (in treecontrolpeer.cxx):
  sal_uInt16 UnoTreeListItem::IsA()
  {
      return 0;
  }
  returns 0 on any IsA() call to a tree item, so the check in 
SvTreeListBox::ImplEditEntry never succeeds.

How to fix it:
  The simplest fix, as proposed in the original bug report, is to make 
UnoTreeListItem::IsA() return SV_ITEM_ID_LBOXSTRING instead of 0. It 
should work because UnoTreeListItem is derived from SvLBoxItem.
  Since i am unable to build LibreOffice from the source, i cannot 
verify that this does fix the issue at hand and that it does not 
introduce new issues.

Testcase is available in the original bug report.