Removed the GTK+ scrollbars in Area thing as that can't be resolved (it's a property of the minimum preferred size of both the GtkScrollbar and GtkScrolledWindow; these can't be changed). Thanks to baedert in irc.gimp.net/#gtk+.
Removed TODO about GetDC() vs GetWindowDC() in prefsize_windows.go; the latter includes the non-client area, which we won't bother with.
Actually make the prefsize_windows.go change from the previous commit.
Removed TODO about messagebox font in stdfont_windows.go; decided to just use it.
Properly integrated the Area preferred size into the Windows backend's preferred size table.
Removed TODO about Windows label ellipsization; SS_LEFTNOWORDWRAP's docs say so. Replaced it with one about vertical label alignment...
Resolved vertical alignment of labels on Windows.
Updated GTK+/Wayland TODO now that we know what's wrong