Add a WX_APPNAME_DATA_DIR hack for wxStandardPaths::GetDataDir().

Add a WX_APPNAME_DATA_DIR hack for wxStandardPaths::GetDataDir().

Applications using wxStandardPaths::GetDataDir() to find their files under
Unix can't be ran without being installed as they look for their data files
under $prefix/share/appname. Make it possible to override this location by
setting WX_APPNAME_DATA_DIR environment variable to allow running them without
installation.

Notice that this shouldn't present any security risk unless the application is
SUID (which would be a very bad idea anyhow).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62337 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775