apprt/gtk: (clipboard) add X11 atoms, extra MIME types for text content

This adds the UTF8_STRING atom and explicit UTF-8 character set MIME
type (text/plain;charset=utf-8) to text content when being sent to the
clipboard under the new multipart support.

This fixes clipboard support under X11 particularly, which generally
looks for the UTF8_STRING atom when looking for text content. This can be
verified with xclip -out -verbose, or trying to do things like paste in
Firefox.

I've noted that there's a number of other older atoms that exist, but
I've refrained from adding them for now. Kitty only seems to set
UTF8_STRING and I've had a hard time finding consensus on what exactly
is the correct set otherwise.

src/apprt/gtk/class/surface.zig

@@ -3364,8 +3364,42 @@ const Clipboard = struct {
                 for (contents, 0..) |content, i| {
                     const bytes = glib.Bytes.new(content.data.ptr, content.data.len);
                     defer bytes.unref();
-                    const provider = gdk.ContentProvider.newForBytes(content.mime, bytes);
-                    providers[i] = provider;
+                    if (std.mem.eql(u8, content.mime, "text/plain")) {
+                        // Add some extra MIME types (and X11 atoms) for
+                        // text/plain. This can be expanded on if certain
+                        // applications are expecting text in a particular type
+                        // or atom that is not currently here; UTF8_STRING
+                        // seems to be the most common one for modern X11, but
+                        // there are some older ones, e.g., XA_STRING or just
+                        // plain STRING. Kitty seems to get by with just
+                        // UTF8_STRING, but I'm also adding the explicit utf-8
+                        // MIME parameter for correctness; technically, for
+                        // MIME, when the charset is missing, the default
+                        // charset is ASCII.
+                        const text_provider_atoms = [_][:0]const u8{
+                            "text/plain",
+                            "text/plain;charset=utf-8",
+                            "UTF8_STRING",
+                        };
+                        // Following on the same logic as our outer union,
+                        // looks like we only need this memory during union
+                        // construction, so it's okay if this is just a
+                        // static-length array and goes out of scope when we're
+                        // done. Similarly, we don't unref these providers.
+                        var text_providers: [text_provider_atoms.len]*gdk.ContentProvider = undefined;
+                        for (text_provider_atoms, 0..) |atom, j| {
+                            const provider = gdk.ContentProvider.newForBytes(atom, bytes);
+                            text_providers[j] = provider;
+                        }
+                        const text_union = gdk.ContentProvider.newUnion(
+                            &text_providers,
+                            text_providers.len,
+                        );
+                        providers[i] = text_union;
+                    } else {
+                        const provider = gdk.ContentProvider.newForBytes(content.mime, bytes);
+                        providers[i] = provider;
+                    }
                 }
 
                 const all = gdk.ContentProvider.newUnion(providers.ptr, providers.len);