Add qml_module to builder and create qmldir and QML module from build script options

[Be-ing]

Register the qml_module in the build script as we need to produce proper QML modules.

Note that we need to generate a qmldir, so for basic ones this will just include the plugin name etc, but if there are QML files included in that module they need to be listed in the qmldir too, so we need this to occur in the build script.

So

• Add qml_module to CxxQtBuilder
• Remove qml_uri and qml_version from qobject macro
• Add qml_element to the qobject macro

This could then look like

#[cxx_qt::qobject(qml_element)
pub struct Thing;

.qml_module("com.foo.bar", 1, 0, |files| {
  files.rs("src/rust/thing.rs");
  files.qml("src/qml/main.qml");
})

Original description

It would be helpful to only need to specify the QML URI and version once in a central place, for example build.rs, to avoid repeating this information for multiple QObjects. QObjects exposed to QML could then be indicated with #[cxxqt::qobject(qml_element)].

#428 (comment)

[ahayzen]

I wonder if we should require always specifying qml_element or qml_singleton (which implies a qml_element) 🤔

As when using this it was odd that specifying a uri and version magically made it an element too (and not obvious if that was then a singleton ?)

[ahayzen-kdab]

For non-QML related files and resources, eg QObjects or resources exposed to just C++, these would still need to use the old .file(...) and .qrc(...) methods outside of the qml_module(uri, version, |files| { ... }) callback.

This is fine and can be documented as such, we should just ensure that the API is the same, eg there is .rs(...) vs .file(...) currently for rust source files should be scanned for a #[cxx_qt::bridge]

[ahayzen-kdab]

This was has been done and the API is the following...

        .qml_module(QmlModule {
            uri: "com.kdab.cxx_qt.demo",
            rust_files: &["src/cxxqt_object.rs"],
            qml_files: &["../qml/main.qml"],
            ..Default::default()
        })