evdevtouch: Enable touch in multi-screen eglfs environments

Parse the touchDevice property from the KMS/DRM config file. When all
outputs have an explicitly specified index in the virtual desktop, we
can set up a mapping between the device node and the screen index. It
is somewhat fragile (device nodes may change, requires explicit
virtualIndex properties for all outputs, etc.) but better than
nothing.

For example, having the screen on DisplayPort as primary and the
touchscreen on HDMI as the secondary screen breaks by default because
touching the second screen generates touch (and synthesized mouse)
events for the first screen.  Assuming the touchscreen is
/dev/input/event5, the issue can now be fixed by setting
QT_QPA_EGLFS_KMS_CONFIG with a configuration like the following:

  {
    "device": "drm-nvdc",
    "outputs": [
      {
        "name": "HDMI1",
        "touchDevice": "/dev/input/event5",
        "virtualIndex": 1
      },
      {
        "name": "DP1",
        "virtualIndex": 0
      }
    ]
  }

Task-number: QTBUG-54151
Change-Id: If97fa18a65599ccfe64ce408ea43086ec3863682
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

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