core: Add support for custom window placement rules

Add support for assigning a window a custom window placement rule used
for calculating the initial window position as well as defining how a
window is constrained.

The custom rule is a declarative rule which defines a set of parameters
which the placing algorithm and constrain algorithm uses for
calculating the position of a window. It is meant to be used to
implement positioning of menus and other popup windows created via
Wayland.

A custom placement rule replaces any other placement or constraint
rule.

https://bugzilla.gnome.org/show_bug.cgi?id=769936