Add qsizetype to cxx-qt crate

[ahayzen-kdab]

This means that when wrapping APIs we don't need to cast the type in C++.

Then once conditional code is working correctly #765 this could mean that cxx-qt-lib can remove a load of C++ code where it's just casting between isize and qsizetype or int depending on Qt 5 or Qt 6.

Consider which of these types from <QtGlobal> could be useful, eg qint64 could remove a lot of static_cast in C++, probably remove the need for C++ trampolines so that there are direct CXX bindings and just have a into/from on the Rust side? (either in a Rust side wrapper or just accept Into in methods not sure what happens with returns though?)

Already map to std numerics (as in Rust/C++ std, so i32 ⇄ qint32 ⇄ std::int32_t)

• qint8
• qint16
• qint32
• quint8
• quint16
• quint32

Most useful types

• qint64
• qintptr
• qsizetype
• quint64
• quintptr
• qreal

Probably useful types?

• qlonglong
• qptrdiff
• qulonglong