Bug fixture for window opening while importing Window class

[Samael-TLB]

Issues addressed/ fixed (known to me atleast):

• #8340
• #8222

Changes done:

• Updated EventLoop.ensure_window function to work with lazy initialization of the Window class
• Converted _initialized to BooleanProperty, changed initialized to read only AliasProperty, added lazy initialization of window Removed side-effects during window import, added lazy initialization of window. This would also help advanced users who want to use multiprocessing with Kivy without side-effects. Removed the bug that Window.initialized was able to be changed/set by users, which should not be the case. Made Window.initialized read-only property.
• Added new conifg value lazy_window_initialization For removing side-effects during Window initialization, while maintaining backward behaviour compatibility, and also for advanced users who would want to have multiprocessing used with Kivy (without multiple window initialization), we added a config value named lazy_window_initialization, which when set would lazily do the window initialization.
• Updated the function setup to properly work with lazy initialization feature of Window
• Updated the fixture kivy_app to properly work with lazy initialization feature of Window

Maintainer merge checklist

• [x] Title is descriptive/clear for inclusion in release notes.
• [ ] Applied a Component: xxx label.
• [ ] Applied the api-deprecation or api-break label.
• [ ] Applied the release-highlight label to be highlighted in release notes.
• [ ] Added to the milestone version it was merged into.
• [x] Unittests are included in PR.
• [x] Properly documented, including versionadded, versionchanged as needed.