Fix imageviewer example

[jwiedey]

As reported in https://github.com/lxn/walk/issues/671, the imageviewer example is broken. This makes it work by:

• Adding a Pages array to TabWidget which contains a TabPage. This keeps the program from crashing.
• Giving that page a name and deleting it when you load the first image, just to clean up the above hack.
• Deleting the ToolBar. It obscured the tab names and wasn't part of the example, so I killed it.