Merging site controller with page controller

This suggestion according to this conversation.

Let’s say you have a site controller (whetever in a plugin or not). Then the user creates a page controller. Kirby will execute the page controller. Without merging those with Shared Controllers manually, the site controller will never executed. Sadly, that you didn’t have possibility to influent all page controller. specially if you working on a plugin.

It would be nice, if the site controller run before page controller every time.
Or better: Make a new Controller (e.g. controller/main.php) that’s executing every time.

Yay. I struggle with this, too. Looking forward to this in 5.