Desktop: Unify save file handling and add file dialog for export (#3008)

* Prepare save file unification

* Desktop add save file dialog

editor/src/messages/frontend/frontend_message.rs

@@ -68,18 +68,18 @@ pub enum FrontendMessage {
 		document_id: DocumentId,
 		name: String,
 		path: Option<PathBuf>,
-		document: String,
+		content: Vec<u8>,
 	},
-	TriggerDownloadImage {
+	TriggerSaveFile {
+		name: String,
+		content: Vec<u8>,
+	},
+	TriggerExportImage {
 		svg: String,
 		name: String,
 		mime: String,
 		size: (f64, f64),
 	},
-	TriggerDownloadTextFile {
-		document: String,
-		name: String,
-	},
 	TriggerFetchAndOpenDocument {
 		name: String,
 		filename: String,

editor/src/messages/portfolio/document/document_message_handler.rs

@@ -1005,7 +1005,7 @@ impl MessageHandler<DocumentMessage, DocumentMessageContext<'_>> for DocumentMes
 					document_id,
 					name,
 					path: self.path.clone(),
-					document: self.serialize_document(),
+					content: self.serialize_document().into_bytes(),
 				})
 			}
 			DocumentMessage::SavedDocument { path } => {