Convert simple popup block to config entity

I suggest creating a new release (4x-dev) for this feature. Because it's a huge change of module that needs to be tested.

This is a great (huge!) patch. I added the incorporation of another patch 🐛 Warning: htmlspecialchars() expects parameter 1 to be string, array given RTBC which is RTBC.

I'm working on a custom module that contains default config for simple_popup_blocks with a dependency on my module. When using 8.x-3.x the config was not being removed when my module was uninstalled. With this patch I get the expected behavior of the config being removed.

Thanks!

One issue that needs addressing is an upgrade path.

On a site with the existing 8.x-3.x module installed, when I patch the module with the code from this MR and run database updates, I see this error on the /admin/reports/status page:

Entity/field definitions

Mismatched entity and/or field definitions
The following changes were detected in the entity type and field definitions.

Simple popup blocks

• The Simple popup blocks entity type needs to be installed.

I've added an update hook inspired by 💬 How to install a new entity type from class annotations in an update hook Active .

I'm not sure about the numbering.