Compatibility with core version 5.8

[carlomanf]

v1.2.0 introduced support for full site editing, with the gutenberg plugin as a dependency. (If the gutenberg plugin was not installed, the legacy funnel builder would persist.)

There are some API changes that will come as a result of changing the dependency from the gutenberg plugin to core version 5.8:

• Instead of checking gutenberg_is_fse_theme(), the plugin should call add_theme_support( 'block-templates' ) and rely on that instead
• The interface for adding a new funnel type needs to be changed, since the core implementation of block templates does not support the post editing screen

v1.3.0 can implement these changes and declare core version 5.8 as its minimum supported version.

I also want a better solution for handling theme switches, which was one of the most difficult parts of version 1.2.0. 7cf7442 handled this by not handling it, i.e. de-activating funnel types upon theme switch, but I quickly changed my mind and implemented a different solution in 6312b4b that re-activates them upon theme switch. I don't like this solution much either, because it breaks if the theme is switched while the plugin is de-activated, so I want to change it again for version 1.3.0.

WordPress/gutenberg#31971 was going to make this task a little bit easier, but it will not be included in 5.8. I am going to explore another idea that I have for handling this more seamlessly, and see whether it's viable.

[carlomanf]

This issue was tagged by mistake in the commit message for 6cf07cd, it should have tagged #15 instead.

[carlomanf]

Closing for now, unless any other outstanding tasks are found.