Hide/show placeholder (instead of remove/append)

[p1gtr]

Currently tagEditor removes the placeholder when hiding it, and re-initiate it when showing it. This makes it impossible to dynamically change the placeholder text (set_placeholder() would reset it every time).

This fiddle shows a solution. I mark all changed code with keyword "TagEditorHack" so just search for it.

Changes needed:

• change set_placeholder() so it only append the placeholder once.
• change all $('.placeholder', ed).remove() to $('.placeholder', ed).hide()
• somehow the changes above cause a racing condition where when you delete a tag, the placeholder would appear before the animation of tag deletion completes. So we also need to change update_globals() so that it only calls set_placeholder() during initialization.