[WIP] xwayland: add incoming drag'n'drop support

[emersion]

Fixes #295

• Figure out how the client knows about the Wayland window position: just draw a window that spans the whole screen
• Figure out how to map X11 protocol to Wayland protocol
• Receive DND_ENTER
• Receive DND_POSITION
• Receive DND_DROP
• Send DND_STATUS
• Receive DND_LEAVE
• Send DND_FINISHED

Test clients:

• Firefox
• GTK+ (under Wayland and Xwayland)
• Chromium
• Gimp

[emersion]

It seems that starting an implicit grab on button down is required to make this work (sway does it, rootston doesn't).

KDE got it working recently: https://phabricator.kde.org/D15627

[L-as]

If I may ask, what happened to this PR?

[emersion]

I wanted to fix Wayland drag-and-drop before working again on this, but to do so I wanted to refactor the data-device code first. This is being done in #1427, but I haven't had enough time yet to finish it.

[panaman67]

@emersion With #1427 merged, Whats the status of this?

[emersion]

wlroots has migrated to gitlab.freedesktop.org. This pull request has been moved to:

https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/841