[WebODF] 2 month performance blitz for webodf

Hi all,

I'm just putting together a quick plan of what I'll be looking at with regards
to performance, to address some queries & requests raised as part of MR#114
(https://gitorious.org/webodf/webodf/merge_requests/114).

As part of this next cycle, my goal is to have editing of a 20 page ODT and
flat ODT with images up to a level that is responsive. At the moment, deleting a
single character at the end of a sample 11 page document produces an extremely
noticable 500ms delay before the character is gone.

>From my initial investigation so far my plan of attack for this is:
1. Improve obviously suboptimal paths:
OdtDocument
* TextPositionFilter - Most of the container checks should be filtered nodes

2. Eliminate the number of times the average run needs to step through
the document to find a position:
* e.g., upgradeWhiteSpace is called at a specific position. Any Op using
this therefore steps through the document up to that specific position usually
2 or 3 times.

3. Implement a bookmark system to quickly retrieve iterators at specific
positions within the document.
I've used one of these internally for several months now at a different layer
above webodf, and have found this to be the most significant improvement.

The plan with each of these is to introduce benchmarking numbers to allow the
performance improvements to be proven. As such, I don't plan on addressing
any of the performance related concerns in MR#114, as they are literally (from
initial profiling checks) drops in a very large ocean of improvement.

If people have other ideas, complaints, etc., as ever, I'm open to anything :)

Cheers,

Philip