wxGTK, wxWidgets: adjust short_desc to reflect version situation

As per gh-520, wxGTK is supposed to track version 2.x and wxWidgets to
track 3.x – Both packages use GTK, so adding "GTK Version" to only one
of the two created a false distinction. Instead, separate them by
version.
wxWidgets: ensure that we build with mediactrl enabled

All other configure args that differ between wxGTK and wxWidgets are
either defaults, deprecated or not wanted anymore (monolithic)
wxGTK: support gstreamer
poedit: build with wxWidgets instead of wxGTK
wxPython: update to 3.0.0.0