don't allow elements to be added without at least an `append`

[beb]

win32, r279:

Shoes.app do
  button "add para" do
    para "here"
  end
end

The text "here" is shown just after the window is resized or moved out of the screen and moved in again.

Expected: should be visible instantly.

[beb]

After clicking the button, of course :)

[why]

Well, the code should be:

Shoes.app do
  button "add para" do
    append { para "here" }
  end
end

Adding the element without an append should throw an error.

[why]

Moved to ditz, new ticket #shoes-1.