add option to record values for x/y plot each time a value changes w/o stopping

[cs01]

Currently the x/y plot of expressions over time only records new values when the program stops and gdbgui receives new values of expressions.

A new option should be added to allow the user to record all changes to values. This could be done using a combination of mi commands:

• first create a watchpoint when the value is written to: -break-watch <expr>
• then add a command to run when that breakpoint is hit: -break-commands # "continue"
  • (need to confirm the updated value is output by gdb and received by gdbgui before continuing, but in theory this should work)

This flow will be used in #184, but probably with the a break-command of "elapsed-time" "continue". Does that sound right @WesleyYue?

[WesleyYue]

I finally had a chance to take a look at the gdb machine interface documentation and I think you're spot on. What you described should work.