Keep track of the `GParamSpec`s of the properties. This allows us to use
`g_object_notify_by_pspec()`, which is a bit more performant than
`g_object_notify()`(as it doesn't need to take a global lock to lookup
the property name). It also prevents accidental typos in the property
name at compile time.

Also always add `G_PARAM_STATIC_STRINGS`, to prevent some unnecessary
string duplications of property name, blurb and description.