Port libgdata from autotools to meson

Libgdata's port to meson is long due, and this MR is an effort to towards that direction. At the moment, a lot needs to be done given how vast libgdata is due to its different services and dependencies.

TODO:

1. Create meson.build for the remaining sub directories and install the headers from those.
2. Edit all the meson.build files to generate object files, executables and libraries.
3. FIX liboauth to be the dependency. For some unknown reason, pkg-config can't find it in my system, albeirt ldconfig shows that it's installed.