build: Require GLib ≥ 2.43.2

We need g_set_object().
area: Use g_set_object()
basicmaterial: Use g_set_object()
directionallight: Add properties

And accessors, where necessary.
mesh: Add properties
shadermaterial: Add properties

And accessors, where needed.
Merge pull request #3 from ebassi/set-object

Use g_set_object()