Added ToolTip support for CDB.
Make debugger tooltip API more general, have the engines check the correct file type, figure out the expression and context from the text editor. Put common functionality in watchutils.cpp. In the CDB engine, check whether a tooltip expression is a known variable within the stack frame context. If so, retrieve via symbol group or dumpers. Cache by function and expression. Reviewed-by: hjk <email@example.com>