Preferences: Fix tests, improve clarity, and more

- Fix sync and advanced preferences tests; use a new waitForFirstPaneLoad()
  functions instead of the old paneload event listener
- Remove empty preferences_searchTest.js
- Rename some Zotero_Preferences members/functions for better clarity and
  public/private differentiation
- Reorder, also for clarity
- Fix tabIndex parameter causing an error if invalid
- Remove window.sizeToContent() call in Sync.displayFields()
    - So *that's* why the window resized every time the sync pane was loaded...
- Deprecate openURL() and simplify openInViewer()

test/tests/preferences_syncTest.js

@@ -3,18 +3,10 @@ describe("Sync Preferences", function () {
 	before(function* () {
 		// Load prefs with sync pane
 		win = yield loadWindow("chrome://zotero/content/preferences/preferences.xhtml", {
-			pane: 'zotero-prefpane-sync',
-			tabIndex: 0
+			pane: 'zotero-prefpane-sync'
 		});
 		doc = win.document;
-		let defer = Zotero.Promise.defer();
-		let pane = doc.getElementById('zotero-prefpane-sync');
-		if (!pane.loaded) {
-			pane.addEventListener('paneload', function () {
-				defer.resolve();
-			});
-			yield defer.promise;
-		}
+		yield win.Zotero_Preferences.waitForFirstPaneLoad();
 	});
 
 	after(function() {