Those property features don't seem to be in use anywhere.
They are redundant since the docs cover the same information
and more. They also created unnecessary translation work.
Closes#4904
Since it's a type with sub-classes, we need to use GTypeInstance (at the
very least), otherwise we won't be able to address each sub-class as
such.
This is similar to how GskRenderNode and GdkEvent are handled, with the
added difficulty that GtkExpression is meant to be used in properties,
in order to be deserialised by GtkBuilder. This requires adding a
GParamSpec sub-class that we can match on from within GtkBuilder,
alongside some convenience API for storing a GtkExpression inside a
GValue.
Users provide a search filter and an expression that evaluates the items
to a string and then the filter goes and matches those strings to the
search term.
