Don't replace unchanged panels

This may also help the use of iframe in builder and preview panels

Let's go

This may also the opportunity of tidying ApiController:

• Move builder and island id to properties?
• Merge update and thirdPartySettingsUpdate ?

So, when do we want to block refresh:

• BuilderPanel & LayersPanel: onRootDrop and onSlotDrop, but only on move (because we need the refresh to get the placeholder replacement when on add)
• InstanceFormPanel: onInstanceFormChange and onInstanceUpdateButtonClick
• ThirdPartySettings panels (UI Styles, UI Skins...)

Other islands triggering HTMX events but which needs to be refreshed:

• HistoryButton (onUndo, onRedo, onClear)
• MenuDelete (onClickDelete)
• StateButtons (onSave, onReset, onRevert)
• Menu (onClickDuplicate, onClickPaste)

Those islands don't trigger events:

• ViewportSwitcher
• ComponentLibraryPanel
• BlockLibraryPanel
• ...

We will also do those non-relaetd tasks:

• Merge InfoPanel into InstanceForm panel
• Add readonly keywords to HistoryStep

Maybe the opportunity of little clean-up

2 things to be careful about:

• If the UIP2 AJAX issue is solved on 1.0.x (to be confirmed), it is not the case in ths MR so it may be a regression
• the last commit Remove previous use of current_island_id is a bit bold with the removal of a badly understood mechanism:

-    if ($form_state->isRebuilding() && (!isset($options['current_island_id']) || $this->getPluginId() === $options['current_island_id'])) {
+    if ($form_state->isRebuilding()) {

if there is any doubt, let's revert it and move the change to a dedicated issue

If we merge it like that, here is the related Dev Tools issue: #3544616: Follow IslandEventSubscriberInterface::onUpdate() changed