Multiple include fix

Excellent find by @jan76. He discovered that the same include directories were being pulled again and again, e.g. http://pastebin.com/0Y4Eiy9y. It turns out that in qmake, ```+=``` adds to a list of variables, where as "the *= operator adds a value to the list of values in a variable, but only if it is not already present." (http://stackoverflow.com/questions/14641929/what-does-it-mean-in-pro-file-qt)

The result looks a lot more sane: http://pastebin.com/j9WkJztw

We were using += across many files. Unfortunately, some of the problems are coming from libqxt, so we're patching an external lib.

EDIT: We could probably do the same thing with DEPENDPATH. I'll wait until this is done before tackling that.