Support QFlags

[jnbooth]

This PR defines a QFlags<T: QFlag> type to address the non-codegen parts of #1123. It mimics the API of Qt's own QFlags, including operators. The one exception is that because Rust doesn't have the ~ operator, the ~ operator in Qt is mapped to ! in Rust, while the ! operator in Qt is mapped to QFlags::is_empty in Rust.

The backing QFlag trait provides two things: a function for converting from the FFI enum type to its underlying representation (i.e. value.repr) and the TypeId of the corresponding QFlags type. For example:

#[cxx::bridge]
mod ffi {
    #[repr(i32)]
    enum MyFlag { A, B }; 
}
pub use ffi::MyFlag;

unsafe impl QFlag for MyFlag {
    type TypeId = type_id!("QFlags_MyFlag");
    type Repr = i32;
    fn to_repr(self) -> Self::Repr {
        self.repr
    }
}
pub type MyFlags = QFlags<MyFlag>;

It's unsafe because it has the same safety contract as cxx::ExternType::Id. There's also an internal utility macro to replace the above impl block with

impl_qflag!(MyFlag, "QFlags_MyFlag", i32);

Since QFlags can now be defined, this PR also adds QGuiApplication::keyboard_modifiers, QGuiApplication::mouse_buttons, and QGuiApplication::query_keyboard_modifiers,

Closes #1123

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (50ca5e2) to head (7615883).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1231   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           73        73           
  Lines        12612     12612           
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  Hits         12612     12612           

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[LeonMatthesKDAB]

Hi @jnbooth thank you for opening the PR on this.

I left some small comments, the approach itself is solid!

[LeonMatthesKDAB]

Excellent work, thank you for the contribution @jnbooth