minor code simplification

snake to camel case fixes in EventHandler

For compatibility reasons with basically all existing plugins, the
methods have not been renamed. Instead the appropriate check is ignored.

Move list of plugin types to plugin controller constant

There is no need to have them in a global variable.

♻️ Don't use deprecated subscription methods

Automatic code style fixes for subscription classes

✅ Fix Subscriptions tests

This splits up and slightly refactors the tests for the refactored subscriptions classes

♻️ Extract subscription classes for PSR 2

This allows us to rename the snake_case methods and to group them in a
more sensible manner.

Avoid accessing the evet system before it's intialized

reimplements cbfa1cdd75e162a1a8fac054453f4554f96e3362

Avoid processing events before the Event System is intiialized

isEnabled instead of isDisabled

Boolean Methods with a negative name are a bit confusing. Getting false
for an enabled plugin? Better check for Enabled status.

removed get_directory() method from PluginController

This method did absolutely nothing and just returned the plugin name.

fix type hints

thos broke during refactoring

moved plugin controller to Extension namespace

removed deleted file from autoloader

deprecated trigger_event() in favor of a static method on Event

First go at moving the plugin classes into their own namespace

some PSR-2 fixes for the Mailer class

The defines might need to be moved elsewhere

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Only ob_flush if there is actually a buffer.

If there is no buffer, application dies with no way to recover.

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