Better use 0 instead of TOOLBOX_ITEM_NOTFOUND for "no ID available"

...return value from ToolBox::GetItemId, as is done in all other places where
such is returned from one of the ToolBox::GetItemId overloads.  And
TOOLBOX_ITEM_NOTFOUND is otherwise only used for item positions, not item IDs.
(This lone odd use of TOOLBOX_ITEM_NOTFOUND was introduced with
ae5f75ab "Allow a common op. on a disposed
toolbox", probably by accident.)

Change-Id: Id767b1d8d0b3cdfdc0ec525f165d75ae0cef7301

vcl/source/window/toolbox2.cxx

@@ -789,7 +789,7 @@ Size ToolBox::GetItemContentSize( sal_uInt16 nItemId ) const
 sal_uInt16 ToolBox::GetItemId(const OUString &rCommand) const
 {
     if (!mpData)
-        return TOOLBOX_ITEM_NOTFOUND;
+        return 0;
 
     for (std::vector<ImplToolItem>::const_iterator it = mpData->m_aItems.begin(); it != mpData->m_aItems.end(); ++it)
     {