Fix format selection when saving a palette in the Electron app

This commit addresses an issue in the Electron app where the file format
selected in the "Save as type" dropdown, or chosen by typing the file
extension, was not respected when saving a palette.
It saved always in GIMP Palette file format, due to the palette formats
not having defined `mimeType` properties.

Color palette formats have virtually no MIME type consensus, but using
the AnyPalette.js format identifiers works fine here.
For image formats, the `formatID` is the same as the `mimeType`, so this
change should have no effect on image file saving.

Steps to reproduce:
- Run the electron app: `npm run electron:start`
- Click Colors > Save Colors
- Select "Sketch palette (*.sketchpalette)" for "Save as type"
- Enter a file name (ending with .sketchpalette) and click Save
- The saved file should match the chosen format (JSON-based for Sketch)

Author: 1j01

src/electron-injected.js

@@ -137,15 +137,15 @@ window.systemHooks.showSaveFileDialog = async ({ formats, defaultFileName, defau
 	if (!format) {
 		return show_error_message(`Can't save as *.${extension} - Try adding .png to the end of the file name`);
 	}
-	const blob = await getBlob(format.mimeType);
+	const blob = await getBlob(format.formatID);
 	const { responseCode, error } = await write_blob_to_file_path(filePath, blob);
 	if (responseCode !== "SUCCESS") {
 		return show_save_error_message(responseCode, error);
 	}
 	savedCallbackUnreliable && savedCallbackUnreliable({
 		// newFileName: path.basename(filePath),
 		newFileName: fileName,
-		newFileFormatID: format.mimeType,
+		newFileFormatID: format.formatID,
 		newFileHandle: filePath,
 		newBlob: blob,
 	});