Allow creating a scope for selected pods

[lihebi]

When the user selects some code pods, we would display a floating toolbar containing the delete and group buttons.

• The delete logic is in #162.
• The group button, once clicked, will create a new scope around all the selected pods. The parent of this new scope will be the original parent of the selected pods.

Note that we (have already) restricted all the selected pods to be of the same parent (in #136).

[forrestbao]

Issue #124 requests this feature and the other way around.