* Added a context menu on the buttons including Clear & Restore Default
* Allow clearing (unsetting) inputs. Added a Clear All button
* Allow restoring a single input to default (instead of all)
This avoids a redundant std::string construction if a key doesn't exist
in the map already.
e.g.
data[key] requires constructing a new default instance of the value in
the map (but this is wasteful, since we're already setting something
into the map over top of it).
zhupengfei