Apparently Microsoft added just recently the feature to install user
font (as opposed to system-wide fonts), without administration rights
(yes, only now, how crazy is that?). Right now GIMP does not see fonts
there.

We have an upstream report at fontconfig where such a default search
path should happen.
See: https://gitlab.freedesktop.org/fontconfig/fontconfig/issues/144
Until it gets fixed there, let's just add the user fonts dir ourselves
in GIMP. This code should get killed later.

Notes:
- I renamed various DEFAULT_* data macros to GIMP_DEFAULT_* because
DEFAULT_PALETTE was conflicting with another macro in Windows API!
- Also I removed the DATADIR macro set under app/config/ because it is
also conflicting and anyway we use it in no files on this level.
- This is not perfectly tested on Windows. Please everyone with Windows
access, could you build and test if it works fine before release?