stateless debugging

[pvmm]

Another way to say the same thing: the breakpoint viewer should (in a way) be "stateless". That is: when you throw away all the information from the breakpoint viewer widget, and then repopulate it from the breakpoint information you query from openMSX (plus the information of the Symbol table in the debugger), the result should be the same as at the start.

Originally posted by @m9710797 in #126 (comment)