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 don't understand why does this need (or if it needs) to be documented here, but this is not processed by Doxygen and so doesn't appear in the generated docs at all. It needs to be mentioned inside `@begin/endStyleTable` tags in the class documentation itself instead (or in addition to...).