Update to wxWidgets 3.1.5

[imciner2]

Description of the problem:

Version 4.1.1 bundles a post-3.1.4 but pre-3.1.5 version of wxWidgets as its internal version. Upstream wxWidgets has released 3.1.5 about a month ago, so it would be appreciated if a new release of Phoenix was made that uses 3.1.5 as its base wxWidgets version.

[imciner2]

Note that it probably needs wxWidgets/wxWidgets#2366 as a patch to build - otherwise almost the entire wxConfigBase class doesn't have doxygen information.

[StefanBruens]

Also #1909

[imciner2]

It probably also needs the following two commits from the docs update PR:

• wxWidgets/wxWidgets@8482b50
• wxWidgets/wxWidgets@c2d176e

[niki-sp]

wxWidgets 3.1.5 has bug in wxBitmap MSW code for masks. wxWidgets/src/msw/bitmap.cpp from 3.1.6 fixes that