Add is_alive method to CxxQtThread

[akiselev]

When combining async Rust channels and QT objects, it's really easy to end up in a situation where a message is received on the channel after the QObject has already been destroyed, causing CxxQtThread::queue to throw an exception. This PR introduces an is_alive() function to check the handle if it's been invalidated.

• Implement is_alive method in C++ CxxQtThread class
• Add is_alive to Rust Threading trait
• Generate FFI bindings for is_alive in cxx-qt-gen
• Expose is_alive as a public method on CxxQtThread in Rust
• Update documentation for Threading trait and CxxQtThread