fx-compat: Add HiddenBrowser.jsm

Remove Zotero.Browser and add HiddenBrowser.jsm. Post-Fission, web/file
content loads in a separate process, so it's not possible (as best as I
can tell) to directly access the contents of a hidden browser -- it just
appears as about:blank in the parent process. We now use Mozilla's
JSWindowActor mechanism [1] to get page data, including character set
and body text for full-text indexing. We'll have to evaluate other uses
of hidden browsers to see how to handle them.

This also adds include.jsm for loading the Zotero object into a JSM.

[1] https://firefox-source-docs.mozilla.org/dom/ipc/jsactors.html

test/tests/HiddenBrowserTest.js

@@ -0,0 +1,39 @@
+describe("HiddenBrowser", function() {
+	const { HiddenBrowser } = ChromeUtils.import(
+		"chrome://zotero/content/HiddenBrowser.jsm"
+	);
+	
+	describe("#getPageData()", function () {
+		it("should handle local UTF-8 HTML file", async function () {
+			var path = OS.Path.join(getTestDataDirectory().path, 'test-hidden.html');
+			var browser = await HiddenBrowser.create(path);
+			var { characterSet, bodyText } = await HiddenBrowser.getPageData(
+				browser, ['characterSet', 'bodyText']
+			);
+			assert.equal(characterSet, 'UTF-8');
+			// Should ignore hidden text
+			assert.equal(bodyText, 'This is a test.');
+		});
+		
+		it("should handle local GBK HTML file", async function () {
+			var path = OS.Path.join(getTestDataDirectory().path, 'charsets', 'gbk.html');
+			var browser = await HiddenBrowser.create(path);
+			var { characterSet, bodyText } = await HiddenBrowser.getPageData(
+				browser, ['characterSet', 'bodyText']
+			);
+			assert.equal(characterSet, 'GBK');
+			assert.equal(bodyText, '主体');
+		});
+		
+		it("should handle local GBK text file", async function () {
+			var path = OS.Path.join(getTestDataDirectory().path, 'charsets', 'gbk.txt');
+			var browser = await HiddenBrowser.create(path);
+			var { characterSet, bodyText } = await HiddenBrowser.getPageData(
+				browser, ['characterSet', 'bodyText']
+			);
+			HiddenBrowser.destroy(browser);
+			assert.equal(characterSet, 'GBK');
+			assert.equal(bodyText, '这是一个测试文件。');
+		});
+	});
+});