Add support for `extern "C++Qt"` with type, fn, and signals

[ahayzen-kdab]

Our current generation is mostly about the extern "RustQt" side, once #557 is done we'll have extern "RustQt" with support for inherit and signals. Then #558 adds support for invokables and #559 for type and properties.

But we also want to support foreign types (eg type QButton), so we need to support extern "C++Qt".

• [x] Add support in parser phase for C++Qt
• [x] Add support in generation phases for a type that is foreign (it only needs to generate CXX bridges and the signal connect parts?)
• [x] Ensure that type QButton;, fn show(self: Pin<&mut QButton>);, #[qsignal] fn clicked(self: Pin<&mut QButton>) work, but note that qproperty, inherit, qinvokable should not work ?
• [x] Change connections to be DirectConnection by default and make choosing a connection type as an unsafe closure (as QueuedConnection could have a mutable reference to something and be executed at any time in the future)

Consider any notes from #527

[ahayzen-kdab]

C++ prototypes of the mixins / templates route

#include #include #include #include #include #include #include using namespace std; using namespace std::chrono; class Locking { public: std::lock_guard lock() { std::cout << "Locking" << std::endl; return std::lock_guard(m_mutex); }; private: std::recursive_mutex m_mutex; }; class Object {}; class MyObject : public Locking {}; template struct MaybeLock { MaybeLock(T &) {} }; template struct MaybeLock>> { MaybeLock(Locking &locking) : m_lock(locking.lock()) {} std::lock_guard m_lock; }; template void lock(Object &obj) { MaybeLock

[LeonMatthesKDAB]

TODO: Check correct marker Traits (Send, Sync, etc.)