Gtk4 migration

[MuhammadMuradG]

Describe your changes in detail

This PR migrates from Gtk3 to Gtk4, from3to4, and taking also into consideration Gtk5, from4to5.

Note the following:

• This changes required GTK4 v4.10 at least (Primary, because we use Gtk.AlertDialog to implement Toga Dialog. Otherwise, we either rebuild it from scratch using Gtk.Window or using deprecated Gtk API).
• Layout mechanism has been completely refactored.
• Applying styles on widgets has been changed.
• Tree widget needs a refactoring by one of the new Gtk4 widgets instead of removed Gtk3 widget (Gtk.TreeModel).
• Menus and Commands will be addressed in HIG-compliant for GTK menus #1931
• Fixes some bugs.

Audit checklist:

• app
• fonts
• icons
• images
• keys
• paths
• window
• activityindicator
• base
• box
• button
• canvas
• dateinput
• detailedlist
• divider
• imageview
• label
• multilinetextinput
• numberinput
• optioncontainer
• paswordinput
• progressbar
• scrollcontainer
• selection
• slider
• splitcontainer
• switch
• table
• textinput
• timeinput
• tree
• webview

What problem does this change solve?

This PR migrate from Gtk3 (have a lot of deprecated functions and old UI) to Gtk4 and does not violate what will be in Gtk5.

Related issues:

Refs #1931
Refs #1935

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

Wow - this is a big chunk of work :-) Thanks for taking on this project.

As I indicated in #1935, we're in the middle of a widget audit right now, so there's a lot of churn happening with widget implementations - but we're also building GUI test cases for all widgets. Those tests (and the test probes in the gtk/tests_backends folder) will be a key part of acceptance testing for this PR.

At present, this PR is failing CI because of basic formatting (towncrier and pre-commit failures) - I'd strongly recommend setting up your development environment as described in the Toga docs to avoid these issues. Once those problems are resolved, you're going to hit a bunch of problems with the testbed app - this is the test harness we're using to validate GUI backends. That testbed will require modifications to run GTK4 code, as will the GTK probes (defined in the gtk/tests_backend folder) that allow the GTK backend to be tested. Procedurally, I'm unlikely to take a deep dive into this code until those tests pass.