Added common utils lib (2).

[lpaulsen93]

The lib is placed in sub-directory 'utils'.
So far it only includes utility functions for scanning a directory for files.

This is a fresh PR only including the changes for the common lib in folder utils. It does not include changes on other plugins. This replaces #620.

[eht16]

I like the idea of this library and besides the mentioned additional use
for "relative bundle path code used on Windows and Mac", we probably can
move some of the plugin configuration config file handling in each plugin
here. There is a lot of duplicated code around.

Since we add a new dependency for other plugins here (currently just Workbench but probably
more in the future), we should emphasize this in the NEWS to help distro packagers when releasing. They probably need to add something like geany-plugins-common package and make all other plugin sub packages depend on this (as it is already on Debian).

Apropos, in order to be able to compile it on Windows I had to add:
-no-undefined to libgeanypluginutils_la_LDFLAGS.
Without, linking fails with:
can't build shared library unless -no-undefined is specified.
I'm not sure whether we should add it there or better in
build/vars-build.mk where it is already specified for AM_LDFLAGS.

[codebrainz]

I also mentioned some weirdness about AM_LDFLAGS in the PR I made for the other/first PR for this, but it's mysteriously missing now. I think it was that it cannot be used by anything but plugins since it puts libtool -module -avoid-version flags in there. That is why the Makefile.am explicitly uses GP_LDFLAGS instead of implicitly inheriting the AM_LDFLAGS.

Also, I left a TODO in the .m4 template that might be nice to fix.