Fixed: paste in Firefox.

The paste into input field hack requires that we don't stop the Cmd-V/Ctrl-V keydown propagation.
Fixed: Safari "can't paste" code triggered even for a content editable target.

These targets might be rare but could occur in some Cappuccino text editor widget, in which case our standard native paste handling from Safari should work fine.
Simplify copy and paste code path.

This change begins to clean up the control flow of copy and paste. Before, copy and paste was dealt with partly in CPPlatformWindow, partly in CPApp and partly in CPTextField.

Now, CPApplication has been removed from the path, simplifying it down to the two more expected actors.
Test: include an image in Copy and Paste test.

Image copy and paste to and from the system clipboard is not available in Cappuccino today. However, the image in the collection view can be used to test internal Cappuccino copy and paste (using Edit > Copy, Edit > Paste from the Cappuccino menu).
Fixed: objects without a superclass.
Fixed: menu items becoming permanently disabled, breaking e.g. Select All.

Without this fix, if a menu item was disabled it could only be automatically re-enabled if the menu item had a `validateMenuItem:` or `validateUserInterfaceItem:` enabled target.

This meant that for example if the Edit > Select All menu item was validated when the first responder was something like a collection view, it'd become disabled. If then the first responder was changed to a text field and revalidation occurred, the menu item would not become enabled and Select All would not be possible in the text field neither through the Edit menu nor the Cmd-A/Ctrl-A keyboard equivalent.

This fix ensures that menu item validation does not only take negative action (disabling enabled items which should be disabled), but also positive action (enabling disabled items which should be enabled), even when there's no `validateMenuItem:` or `validateUserInterfaceItem:`.
Cleanup: TableTest/Editing.
Refactor, restructure and simplify copy and paste handling.

This change brings nearly all copy and paste related into a single, self-contained class. This new class is still somewhat coupled to CPPlatformWindow+DOM.j in the sense that it's highly specific and meant to be called in a very specialised way. Despite this, it's still a significant decoupling and the new code is more readable and easier to follow.