Provide hook to alter available blocks for IPE

Works great but after removing some items the array key order needs to get restored. e.g. array_values($data); not sure if this should be part of the patch.

Re-rolled the patch.

Realized this needs API documentation as well in panels_ipe.api.php

Documentation.

Renamed a variable but forgot to update all occurrences.

This, in theory works for me. And it didn't seem to break anything. Committed.

Just a quick note to not forget to reassign keys in $blocks array, if you unset any block before end of the list (blocks which are not at the end of an array). Otherwise the blocks wont be rendered in IPE. I have to put the following line before end of alter hook:
$blocks = array_values($blocks);

I guess the response has some kind of problems if you unset key in indexed array and end up with something like this

[
  0 => $block1, 
  1 => $block2, 
  3 => $block4
]

In this example I unset block with index 2.

Probably the best way would be to put the fix directly after tiggering alter hooks. So we don't need to reassign keys in alter hooks. We would end up with the:

    // Trigger hook_panels_ipe_blocks_alter(). Allows other modules to change
    // the list of blocks that are visible.
    \Drupal::moduleHandler()->alter('panels_ipe_blocks', $blocks);
    // We need to reindex keys in indexed array so they are properly rendered
    $blocks = array_values($blocks);

@hideaway Done - this was brought up in #2 but never addressed. I committed a quick follow-up based on your comment. Thanks!

I just posed a follow-up to this issue to add some useful context to the hook:

#2886230: hook_panels_ipe_blocks_alter() should receive the PanelsDisplayVariant being editted