Simplify and correct setting of initial directory for wxFilePickerCtrl.

Simplify and correct setting of initial directory for wxFilePickerCtrl.

Create wxFileDialog with correct initial directory and default file name from
the very beginning instead of updating it later. This makes the code simpler
and also actually makes it work as the initial directory setting was
overwritten by setting the default path later even if it didn't contain any
directory component.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72451 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775