Merge wxTaskBarButton into Core

This is a pull request to allow comments and improvements to wxTaskBarButton before merging to core.

See this article for feature details:
https://github.com/zhchbin/wxWidgets/wiki/SOC2014_TASKBAR

I have taken Chaobin's work, rebased it on latest master making a few changes in preparation for this merge:

- I've scrubbed all Bakefile generated files from these commits for easier review (and updated commit messages to reflect that). They'll be re-baked again prior to merge. This means testers should install Bakefile, and generate those files for testing.
- I've appended "Author: Chaobin Zhang" to all commit messages since when this is committed, SVN will scrub the commit credits.
- I've removed the `.gitignore` addition in his original work. This will be handled separately, see [#16473](http://trac.wxwidgets.org/ticket/16473).
I wonder why do we need to have so many friends here, it would arguably be cleaner to use (semi-)public accessors.