Cleaned up paste target and surface styles

Resolves a TODO related to the paste target div.

VisualEditor.php
* Added link to ve.Surface.css

ve.Surface.js
* Cleaned up initialization of .ve-surface element
* Removed float clearing div (see ve.Surface.css)
* Removed paste div (moved to ve.ce.Surface)

ve.Surface.css
* Added rule which adds an ":after" element to handle clearing floats

ve.ce.Surface.js
* Cleaned up initialization of DOM elements
* Added paste target div
* Replaced paste element selectors with direct references to this.$pasteTarget

ve.ce.Surface.css
* Changed paste div styles to use a namespaced class instead of a generic ID

demos/ve/index.php
* Added link to ve.Surface.css

Change-Id: Ib93d45ac82ae643fc8e659f5a063c02a8ddacdde
Merge "Cleaned up paste target and surface styles"