Upstream/pico3

[derMart]

As discussed in https://github.com/nextcloud/cms_pico/issues/218 here is a pull request to upstream to Pico 3.

I had to remove incompass/coverage composer package as this was causing incompatibilities with current symfony packages. Maybe an alternative need to be found for this. However, as I did not get tests to execute anyway (see https://github.com/nextcloud/cms_pico/issues/231) this might be fixed in the future anyway.

[harrysummer]

API_VERSION_4 also needs to be added here:

https://github.com/derMart/cms_pico/blob/7787b36318e471bc59ca14261823933a1374a79a/lib/Model/Theme.php#L51

[derMart]

@harrysummer thanks alot! I've added the change