[UX] Layout UI: Individual tempstore for each block (instead of the global, per-from one we currently have).

We should change the tempstore to save changes on a per-block level so that users can undo changes made to different blocks individually. Consider the following use case: user changes the settings for block 1, block 2 and block 3, then realizes that they need to undo the changes made to block 2. The only way to do that currently is to undo all changes, and then have the user recreate the changes for block 1 and block 3. Really bad UX and it gets even worse as the number of changed blocks increases.

How about we have per-block tempstores instead of a single one for the entire form? That would allow us to place "Discard changes" links individually on each changed block in addition to the global "Cancel" button that we have for the entire form.