A profile is a list of features set to required/optional/disabled
which governs which components are getting loaded, given a static
components list with well-defined dependencies
Each component can now list required and optional dependencies,
using the component feature names to match other components.
In addition, each component feature can be declared as required, optional
or disabled, making optional components easier to deal with.
The component flags (ifexists, nofail) have been removed.
Using virtual components, this system also allows easier customization
of which components should be loaded for a specific configuration,
without requiring the user to copy the list of components and edit it.
Also bump the required glib version to 2.68 for g_assert_cmpstrv()
Each component can optionally "provide" a feature, which is basically
a string that describes the feature (ex. "support.dbus"). If the
component loads successfully, the feature is marked as provided and
can be tested for its presence with wp_core_test_feature()
Do this in wp_core_load_component() and let the component loaders worry
only about creating the object.
Also run the main loop in tests while loading components, to ensure
that the async operation finishes before continuing execution. GTask
makes sure to make the operation async always, by emitting the callback
from an idle GSource.
