Plugins: Add support for loading application chrome and note CSS from the plugin

packages/lib/services/plugins/api/JoplinWindow.ts

@@ -0,0 +1,45 @@
+import Plugin from '../Plugin';
+import * as fs from 'fs-extra';
+
+export interface Implementation {
+	injectCustomStyles(elementId: string, cssFilePath: string): Promise<void>;
+}
+
+export default class JoplinWindow {
+
+	private plugin_: Plugin;
+	private store_: any;
+	private implementation_: Implementation;
+
+	public constructor(implementation: Implementation, plugin: Plugin, store: any) {
+		this.implementation_ = implementation;
+		this.plugin_ = plugin;
+		this.store_ = store;
+	}
+
+	/**
+	 * Loads a chrome CSS file. It will apply to the window UI elements, except
+	 * for the note viewer. It is the same as the "Custom stylesheet for
+	 * Joplin-wide app styles" setting. See the [Load CSS Demo](https://github.com/laurent22/joplin/tree/dev/packages/app-cli/tests/support/plugins/load_css)
+	 * for an example.
+	 */
+	public async loadChromeCssFile(filePath: string) {
+		await this.implementation_.injectCustomStyles(`pluginStyles_${this.plugin_.id}`, filePath);
+	}
+
+	/**
+	 * Loads a note CSS file. It will apply to the note viewer, as well as any
+	 * exported or printed note. It is the same as the "Custom stylesheet for
+	 * rendered Markdown" setting. See the [Load CSS Demo](https://github.com/laurent22/joplin/tree/dev/packages/app-cli/tests/support/plugins/load_css)
+	 * for an example.
+	 */
+	public async loadNoteCssFile(filePath: string) {
+		const cssString = await fs.readFile(filePath, 'utf8');
+
+		this.store_.dispatch({
+			type: 'CUSTOM_CSS_APPEND',
+			css: cssString,
+		});
+	}
+
+}