Layout UI: The "dirty" flag should not be triggered when a user cancels any edit/add

[klonos]

Steps to reproduce:

1. Edit any layout.
2. Click "Add block" in any region, or "Configure" in any existing block.
3. Expand the "Visibility conditions" fieldset, and click on "Add visibility condition".
4. Change your mind and either click "Cancel" or the x button.
5. Hit refresh

Actual behavior:

There is a "dirty" form message.

Expected behavior:

There should not be a "dirty" form message 😄

PS: this is related/similar to #1722 and #1622 ...and if this issue gets a number ending in 22, this will get really weird 😛