Add a button to reset a variable to its standard value

[nbarrientos]

This patchset adds a button to Helpful buffers displaying non-callables (variables) to reset the value to its standard value when the current value differs.

After acting on Reset:

[nbarrientos]

Added a wee test :)

[nbarrientos]

Is there anything else I can do to make this more appealing? 😇

[nbarrientos]

@Wilfred ping

[nbarrientos]

Ping.