Use gdk_screen_get_monitor_plug_name to provide more consistent monitor names on wayland

[dragonnn]

Description

This fixes issue #1086 and #875 (at least partly).
It is based on the linked ags issue in 1086. Yes it is a bad workaround with does require unsafe, pulling in gdk-sys and using a deprecated function from it but really without it eww is basically not usable on wayland with any kind of dynamic monitor setup (common when using laptops).

Usage

Allows using monitor plug names under wayland like that:

(defwindow bar-DP-2
  :monitor "DP-2"
  :geometry (geometry :width "100%"
    :anchor "bottom center"
  )
  :exclusive true
  :namespace "eww"
  (bar)
)

Additional Notes

Even that it uses unsafe, deprecated functions and gdk-sys I really think this is worth adding since as far I have researched this topic they is no other way of improving wayland monitor support other then rewriting to gtk4 with I suspect doesn't happen soon.
Tested it on niri windows manager and works fine.

Checklist

Please make sure you can check all the boxes that apply to this PR.

• All widgets I've added are correctly documented.
• I added my changes to CHANGELOG.md, if appropriate.
• The documentation in the docs/content/main directory has been adjusted to reflect my changes.
• I used cargo fmt to automatically format all code before committing

[w-lfchen]

i've spent the past 4 hours looking into a better implementation for this but i sadly have to give up due to my lack of knowledge on this topic.
i think there is some way using the wayland_client and wayland_protocols crates, we need to get the compositor to list its outputs somehow.
i think this protocol is the one that we need to use, but i'm unsure about whether i'm right or whether this is even possible.
the name property is the one we're looking for.

maybe someone with more experience can pick this one up.