MSVC builds: Add instructions on building with Font Tweaking support

This tells people how the Font Tweaking support that was added for
GTK+-3.24.x can be enabled with the Visual Studio projects, which is
otherwise not enabled by default.
4 jobs for wip/fanc999/fontchooser.win32.gtk3 in 17 minutes and 25 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #275501
fedora-autotools

00:10:57

passed #275502
fedora-meson

00:08:24

manual #275504
win32 allowed to fail manual
msys2-mingw32-autotools
passed #275503
win32
msys2-mingw32-meson

00:17:23