Create Pythonic API to load Slint components

[tronical]

The objective of this task is explore the possibility of providing dynamically generated types for “.slint” files. Beyond classic objects with properties, we can experiment with dynamic imports like in the popular “sh” module. We should review this API with experienced Pythonistas in public discussions.

This work will be sponsored by the NLnet foundation.

Edit: The API review makes most sense after the documentation is up and the rest of the project is ready for review, so let's exclude that from the scope of this issue and include it in the before the final release.

The basic API that's implemented now can be summarized like this:

• ".slint" files are loaded at run-time, either via slint.load_file or via import name_of_file_slint (_slint suffix triggers).
• The loaded file is a dict that provides a class with the name of the component exported in the .slint file.
• Instantiating the class provides an object that exposes the Slint properties, callbacks, and globals.
• Callbacks can be associated by assignment (instance.my_callback = self.foo_function) or by subclassing and using the @slint.callback decorator on a method.
• Data models are sub-classes of slint.Model and must provide row_count, row_data(row) and set_row_data(row, data) functions (the latter only if the model is writable). A convenience slint.ListMode() class exists that wraps a list and provides ways of mutating it (and notifying Slint views accordingly)
• Timers are exposed as slint.Timer.
• Colors and Brushes have slint.Brush and slint.Color wrappers.
• For images there's slint.Image with load_from_path and load_from_svg_data static methods for creation, and path/size properties.

An example of all this in action can be found in https://github.com/slint-ui/slint/blob/master/examples/printerdemo/python/README.md

Overall README: https://github.com/slint-ui/slint/tree/master/api/python

[tronical]

The initial load_file function is added, which exposes instances as objects with properties and callbacks (as on-prefixed properties). The dynamic import is missing still.

[tronical]

One way for the dynamic import would be to allow for writing an __init__.py stub that associates a directory with a specific .slint file, and then allow for writing:

Support ui/__init__.py referenced ui/main_window.slint:

from ui import MainWindow

window = MainWindow()
window.show();

[tronical]

Without __init__.py a neater way might be to add a finder/loader to sys.meta_path and support import foo, which loads foo.slint.

Example:

foo.slint:

export component MainWindow { ... }

blah.py:

import slint
import foo

main_window = foo.MainWindow();
main_window.run()

(this would also work of course with import foo as bar or import MainWindow from foo)

[tronical]

I've updated the description with a summary of the implemented API. Closing this as the initial port being done. Documentation is still missing, but that's part of a milestone that was agreed on separately.