Since FLTK 1.5 building FLTK in the source tree is prohibited by our
CMake setup. Therefore all build artifacts are stored in the build
tree and don't need to be "ignored" in the source tree.
This simplifies the .gitignore files significantly.
There are some exceptions though, for instance .cxx and .h files
generated by fluid which might be created by a user/developer
executing fluid in the source tree.
Move git exclusions from the main .gitignore file to the respective
directories to simplify the main file.
Note to devs: some file types are ignored "everywhere" (e.g. *.exe),
there's no need to add these files to the subdirs.
