filter: Make subfilters a queue

Having subfilters as just two left/right values makes it impossible to
have filters with more than 2 subfilters.

But it would be very nice if we could have a single filter like:

    foo=1 AND bar=1 AND baz=0

Using a queue for the subfilters makes this possible... and it's coming
in a future commit.

https://bugzilla.gnome.org/show_bug.cgi?id=740872
filter: Add new _full variadic constructors

This allows AND-/OR-ing more than 2 subfilters at a time.

A unit test is also added.

https://bugzilla.gnome.org/show_bug.cgi?id=740872
filter: Add new _fullv constructors

This is similar to the ones introduced in the previous commit, but the
argument is a NULL-temrinated array of GomFilter, rather than using
variadic functions.

A unit test is also added.

https://bugzilla.gnome.org/show_bug.cgi?id=740872
filter: Factorize some code

https://bugzilla.gnome.org/show_bug.cgi?id=740872