Add a spinner while the page tree is being modified

Since the version 12 upgrade, our client and their editors have encountered a problem with the page tree. Actions like deleting and moving items aren't immediately reflected in the user interface. This leads to users repeating the action, only to receive an error message. Upon investigation, it appears that deleting triggers a call to /EPiServer/cms/Stores/contentstructure/, which in turn causes a refresh of the page tree. If the call, by whatever reason, is slow the user interface won't have any indication about the process that is happening in the background.

This feedback recommends an addition of a spinner while awaiting for calls that are being made by operations on the page tree.