Refactor all onClick methods so that they can be hijacked

[simon]

For example if you try to hijack function onClickTag(e) you can't because window.event doesn't survive the hijacking. So any plugin writer who wants to customise the behaviours must replace the entire function. Better to refactor so that all onClick* methods just call another method that can be hijacked. Example:  http://trac.tiddlywiki.org/tiddlywiki/browser/Trunk/contributors/SimonBaird/mptw/trunk/core/ExtendTagButtonPlugin.js