Replace renderingEnabled toggle with lock counter

modules/ve/ce/ve.ce.Surface.js
* remove methods enableRendering/disableRendering/isRenderingEnabled
* add methods incRenderLock/decRenderLock/isRenderingLocked
* Consequential method call changes
* try-finally blocks around incRenderLock/decRenderLock pairs

modules/ve/ce/ve.ce.ContentBranchNode.js
* Consequential method call changes

Change-Id: Icb71bf536c7bad3a6c0c6512a2efb9565b5bd086
Merge "Replace renderingEnabled toggle with lock counter"