Support for multiple touchscreens using eglfs

Currently when connecting two touchscreens to a system and using eglfs as the platform plugin you might be able to show a UI on both screens, but the touch input of both screens is always mapped to the primary screen.

Checking on how eglfs, the libinput-plugin and the evdevtouch-plugin are playing together, it seems there is not configuration or autodetection, which touch-input area belong to which screen.

This task is about adding a way for automatically mapping the touch devices to the right screens and to add a configuration option similar to the kms json file for configuring the mapping.