qt: Revamp ClientModel code to handle core signals

Summary:
This is a pure refactoring with no behavior change:

• gets rid of QMetaObject::invokeMethod() "dynamic" calls, i.e., without compile-time checks of a called function name and its parameters
• replaces std::binds with lambdas, making parameter permutation (including parameter omitting) explicit
• makes code simpler, more concise, and easier to reason about

This is a backport of core-gui#581

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15131