An online manual that doesn't add dependencies maxima doesn't have

This patch adds an offline manual to the wxMaxima sources. The manual currently doesn't describe all of wxMaxima's unique features but only all features my colleagues didn't spot at once. But I assume that it will be extended in a next step. Following the one patch for one feature philosophy the help menu entry would be a separate patch.
Since most of this patch was directly copied from maxima it doesn't use any external program maxima doesn't need in order to be built.