Provide hook to alter available layouts for IPE

+++ b/panels_ipe/src/Controller/PanelsIPEPageController.php
@@ -166,12 +166,12 @@ public function getLayouts($panels_storage_type, $panels_storage_id) {
+    $definitions = $this->layoutPluginManager->getDefinitions();

@@ -180,8 +180,12 @@ public function getLayouts($panels_storage_type, $panels_storage_id) {
+    \Drupal::moduleHandler()->alter('panels_ipe_layouts', $layouts);

Hm, so I envision that every different implementation of layouts will want a hook like this. But that would only make sense if you alter the definitions directly, not this meta version.

Also I could see adding a method to LayoutPluginManager that would be like ->getDefintionsForUseCase($use_case) (but a better name) where you would pass in "panels_ipe" and it would be passed along to an alter hook or event. (That should not block this IMO)

It'd be really great if some of the context in which the IPE was being used was passed to this alter hook. Using even just the PanelsDisplayVariant it's possible to learn a lot! For example, you can include some Panelizer specific code in your alter hook to see what content type is currently being edited.

Patch has a small error most likely due to copy-paste from similar issue :)
this:

$blocks = array_values($blocks);

should be:

$layouts = array_values($layouts);

@DamienMcKenna - Would this cover a use case where a site builder writes 2 or 3 custom layouts and wants to restrict users to only those layouts?

version 4.1 reroll

Updating version to 8.x-4.x-dev

Per my comment in #7, here's a new patch that also passes the PanelsDisplayVariant. This could allow you to, for example, change the layouts differently for two particular Panelizer content types.

These are the same changes as in #2886230: hook_panels_ipe_blocks_alter() should receive the PanelsDisplayVariant being editted for hook_panels_ipe_blocks_alter()

Committed! The review in #3 was never really addressed but I feel like that could be a core issue about plugins in general.