updating the local variables may hang gdb

In the program I'm trying to debug there's a local variable which is a class instance with a static member of the same type. This causes gdb to enter an endless loop whenever it is displayed (e.g. with `info locals` or when generating the response to `-stack-list-variables --simple-values`), and this causes gdbgui to hang in turn until I send `SIGINT` to gdb.

I have two suggestions:

+ a workaround: make gdbgui only issue the `stack-list-variables` query when the locals pane is not collapsed. This way I'll still be able to step through the code by hiding the locals.

+ a possible fix: use `-stack-list-variables --no-values` to query the locals and function args and then create a var for each (i.e. start "watching" all locals) - this seems to work when I do it manually... (mmm, you'd probably need to remove all the watched locals and re-add them every time the gui is updated)