Add support for Yes/No/Cancel buttons

[Adanos020]

Fixes #81

I based this off of this fork: https://github.com/zphixon/rfd

The Yes/No/Cancel provided by the OS seem to work fine on Linux and macOS, however:

• Didn't compile on Windows,
• There was no support for custom text in the 3 buttons.

I addressed both of these issues and updated the documentation of MessageDialog accordingly.

Tested on Windows and Linux (GTK3).

WASM, macOS and Linux (XDG) need testing.

[Adanos020]

Looks like I forgot about WASM, will investigate that soon.

[Adanos020]

I fixed all compile errors. There's still one failing GitHub Action for some reason, but it seems unrelated to my changes.

[PolyMeilex]

Cargo deny CI step got fixed, should works as expected after rebase

[Adanos020]

Conflicts resolved and merged.

Edit: I messed this up, undoing.