xwayland: Only pass actual mime type offers to Wayland clients

Fabian Vogtrequested to merge
work/fvogt/xwlselection into master

Wayland clients shouldn't see any ICCCM internal target atoms, only actual mime type offers.

Weston hardcodes support for the UTF8_STRING atom only and wlroots does it like this patch, filtering out anything without / and skipping TARGETS as well as TIMESTAMP explicitly (https://gitlab.freedesktop.org/wlroots/wlroots/-/blob/84fc6aaf5a869da385dc8cf26cc31f4e43f5098c/xwayland/selection/incoming.c#L354).

KWin debug console after selecting text in konsole -platform xcb without this patch:

Screenshot_20250405_222156

With this patch:

Screenshot_20250405_222303

(CC @broulik, came up in a discussion in #plasma)

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