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.
2 jobs for wip/fanc999/fontchooser.win32.gtk3 in 10 minutes and 23 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #230911
build

00:10:23

manual #230912
win32 allowed to fail manual
msys2-mingw32