Try different algorithm to avoid insane xml tag regex

This came about from #433. I don't know much about Eclipse's API, so what I have may very well be wrong or not working. I tried to copy as much as I could understand. Note that I did not do a build. I don't know how to build the project etc. Just wanted to write down my idea and let people in the know see if there is a simpler way to achieve this.

Notable changes:
- Removed public methods: getUnbalancedOpenTag and getUnbalancedClosingTag. Not sure if needed, but they weren't needed for the algo that I had in mind.
- Removed most private methods.
- Renamed endTag variable to closeTag for consistency.
- Look for close tag first, as that is easier to parse for tag name, then search backwards for matching open tag.