Fix: Constructors no longer fail silently

[BenFordTytherington]

• Closes Missing impl Constructor<()> for ... {} in the bridge silently fails #1233
• A shim trait is used to check whether the user declared a constructor
• This is implemented by the bridge if you add the constructor trait in bridge
• This will then allow you to implemented constructor for your type
• This prevents constructors not being used silently

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (96d3233) to head (080cb5f).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1295   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           75        75           
  Lines        12772     12787   +15     
=========================================
+ Hits         12772     12787   +15     

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[LeonMatthesKDAB]

This trait will need the same generics and type arguments as the constructor trait, so that we can make sure you actually declare the correct constructors inside the bridge.

Otherwise you could do stuff like:

#[cxx_qt::bridge]
mod qobject {
    // implements ConstructorDeclared
    impl cxx_qt::Constructor<(i32,)> for MyObject {}
}

impl cxx_qt::Constructor<(i32,)> for MyObject {
   // ...
}

// Additional constructor declared
// No compiler error!
impl cxx_qt::Constructor<()> for MyObject {
  // ...
}

and the trait bound would not catch it.