Walking one step at a time

[tabinohito]

This PR allows the step at a time walking mode as follows:

[mmurooka]

Thanks for adding new features and sharing a nice video :+1:

However, for the following reasons, I think it is better for now that this patch is used in your branch, not merged into the main branch.

• I don't know how common the step-by-step functionality is requested. To avoid over-complicating the BWC code, I think that a separate branch of BWC or a separate controller that inherits from BWC is sufficient for less common functionalities.
• Even if step-by-step functionality were to be added, I think a simpler implementation would be possible. Without making any changes to the FootManager, it would be better to simply call appendFootstep in the GUI callback of the dedicated state. (Note that in velocity mode, on the other hand, this approach is not possible because the target foot position must be changed each cycle.)

So, let's close this PR for a while and review the implementation when it becomes clear that this feature is generally required. Thanks again for your contribution!