Move tooltip-request handling from DebuggerPlugin into
DebuggerToolTipManager.
Request tooltip only if position changed and close
standard tooltip on success.
Replace old debugger tooltip by a new ToolTipManager which
has a list of AbstractDebuggerToolTipWidget with the functionality
to 'acquire' an engine (display its data) and 'release' it
(store engine data and display them as 'previous') and serialization
to XML session data.
DebuggerTreeViewToolTipWidget implements AbstractDebuggerToolTipWidget
for tree model acting as a filter on watch models.
Rubber-stamped-by: hjk
