libdeng2|Script: Utilities for easier creation of native script functions

Binder automates the tasks for registering and unregistering native
entry points that appear as script functions.
Refactor|Client|libgui: Use Binder to manage script bindings
Refactor|Client|libgui: Use Binder to manage script bindings

The native implementation of the functions doesn’t need to check
for argument count as that is already checked earlier when the
call is made in the script.
Fixed|All Games: Errors in release build

Log entries were being printed with debug-only variables or functions.