Routing: Implementing routing stages

### The issue
The current system only allows to modify the URL _before_ it is send to the component router. This means that for example the language filter plugin has to add the SEF prefix before the component router is even started and also has to delete the language value from the URL before that time. That means, that that data is not available for the component router. This is a more general problem that our routing system is very inflexible.

### The proposal
This PR implements a 3-stage processing for the URLs: preprocessing, main stage and postprocessing. This allows the language filter plugin to add the SEF prefix in the preprocessing stage and to delete the language query parameter in the postprocessing stage, making that data available to the component router in between. At the same time, this introduces deprecation notes for all the methods that should be handled as rules in this 3-stage process.

### Backwards Compatibility
This PR should be backwards compatible. There are 4 methods that have been changed, the methods to attach rules and the methods to process these rules. Since the new, second parameter is optional, existing code should not create any issues. Existing rules are still stored in their respective arrays and only the pre- and postprocessing steps add new keys to that array, which would otherwise be ignored. That said, no old software should be affected by these changes. The position of the main stage for parse and build, which represents the rules that we have so far, does not change either, which should prevent any changes in our intermediate results.

### Personal thoughts
All the code that is in the methods around these process*Rules methods should go into a rule and be pushed into the specific stage. The problem is that that change WILL change the way our routers behave towards third party software. That is most likely a break in backwards compatibility, which would force us to postpone that step until Joomla 4.0, which frustrates me immensely...
Another thing that bugs me personally is, that we call the main stage '' in the method parameters, which looks bad. However that makes the nicest implementation from my perspective.

### How to test
Testing here means that nothing changes between applying these changes. So please first check that your site works fine, then apply the change and see that it still works fine.

This PR needs a review by a maintainer.

This was made possible through the generous donation of the people mentioned in the following link via an Indiegogo campaign: http://joomlager.de/crowdfunding/5-contributors
I decided against that, since you can basically extend the current router and have more than those 3 stages if your application needs it. Basically, you could define your own stage or rule group here. If you still feel like this should be checked against, I will put it in.