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

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 `
* 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 https://github.com/cs01/gdbgui/issues/184, but probably with the a break-command of `"elapsed-time" "continue"`. Does that sound right @wesleyyue?