wayland: register drag & drop events

[PeakKS]

~~I tried implementing drag & drop on Wayland, though I don't think it is possible with the current PathBuf based API. The smithay client toolkit provides only a readpipe to get the file content from, and afaict there is no way to get the path from it.~~

Tried a little harder and got it to work, still work to be done e.g. handling multiple file selections.

Related: #1881 #1550

• [x] Tested on all platforms changed
• [ ] Added an entry to the changelog module if knowledge of this change could be valuable to users
• [ ] Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• [x] Created or updated an example program if it would help users understand this functionality
• [ ] Updated feature matrix, if new features were added or implemented

[kchibisov]

See https://github.com/rust-windowing/winit/pull/2429

it all should still apply.

[PeakKS]

I think the functionality is correct now.

[zinifier]

I haven't made deep testing but in a simple application based on libcosmic with this patch applied, dropping a file on the window works on wayland too! Thanks!

I did not review the code before applying.

[markcda]

Guys, any ping on this?