Test for headers and library for FONTFORGE_ARG_WITH_LIBPNG()

Replaced pkg-config check with a check for headers and library.
./configure does a hard-stop if user specifies '--with-png' and there is
no library or developer files available. Fixed *.pc dependency list.
Missed adding to test != xno
Implement the heavy new U. F. O. name mangler.
Add uthash.
Include the submodule mapping.
Fix a compound option.
Fix many things.
Fix path error in submodule location. (How did submodule add work?)
Fix a path for Travis, maybe.
More Travis.
Replace space with tab.
Remove dead comment.
Move glif_name hashing into a separate file and use it in featurefile.c in order to speed fea_classesIntersect.
Fix include listing for dist.
Fix a few things as pointed out by @jtanx.
Fix tabs/spaces.
Fix more.
Restructure the naming code.
Continue adding support for U. F. O. layer names.
Fix compilation problems.