GDK W32: Always set gtk-font-name to the active UI font. Fixes #1484
This makes apps use "Segoe UI 9" by default instead of whatever matches "Sans 10". It also cleans up the code and uses some new pango API while at it.
This was previously disabled in 9e686d1f because it led to a poor glyph coverage on certain versions of Windows which don't default to "Segoe UI 9" (Chinese, Korean, ..) because the font fallback list was missing in pango.
This is about to get fixed in pango!34 (merged) so enable it again when we detect a new enough pango version.