Add the ability to create empty contentEditable windows in MiniBrowser

Add the ability to create empty contentEditable windows in MiniBrowser
https://bugs.webkit.org/show_bug.cgi?id=166801

Reviewed by Wenson Hsieh.

* MiniBrowser/mac/AppDelegate.h:
* MiniBrowser/mac/AppDelegate.m:
(-[BrowserAppDelegate createBrowserWindowController:]):
(-[BrowserAppDelegate newWindow:]):
(-[BrowserAppDelegate newEditorWindow:]):
(-[BrowserAppDelegate openDocument:]):
(-[BrowserAppDelegate _updateNewWindowKeyEquivalents]):
* MiniBrowser/mac/BrowserWindowController.h:
* MiniBrowser/mac/BrowserWindowController.m:
(-[BrowserWindowController loadHTMLString:]):
(-[BrowserWindowController toggleEditable:]):
* MiniBrowser/mac/MainMenu.xib:
* MiniBrowser/mac/WK1BrowserWindowController.m:
(-[WK1BrowserWindowController awakeFromNib]):
(-[WK1BrowserWindowController loadHTMLString:]):
(-[WK1BrowserWindowController validateMenuItem:]):
(-[WK1BrowserWindowController setEditable:]):
(-[WK1BrowserWindowController updateTitle:]):
* MiniBrowser/mac/WK2BrowserWindowController.m:
(-[WK2BrowserWindowController awakeFromNib]):
(-[WK2BrowserWindowController validateMenuItem:]):
(-[WK2BrowserWindowController setEditable:]):
(-[WK2BrowserWindowController updateTitle:]):
(-[WK2BrowserWindowController loadHTMLString:]):
Merge more functionality from WebEditingTester.
Add two menu items for creating empty editable WebKit1/2 windows.
Add a menu item to the Edit menu to toggle editability on any window.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@210483 268f45cc-cd09-0410-ab3c-d52691b4dbfc