Implement possibility to apply changes immediately with commit() instead of apply()

[felixdivo]

Existing users will not need to change anything: It is entirely opt-in, without the need for any more options to be set when defining the properties.

This feature is required in some multithreaded projects in order to avoid race conditions.

[felixdivo]

Ahh, I should've applied this patch to PreferenceHolder ... 😤 Do they depend on each other so that this PR makes sense?