build-sys: use a libcommon.a library to fix tons of build warnings

The same Vala file can't be used several times, as automake will
generate the same target to compile the Vala file.

Instead, compile those shared file once in a common library.

Also update the Vala files to remote the warnings (make classes and
interface public)

https://bugzilla.gnome.org/show_bug.cgi?id=681246