Added placeholder support for tile loading

[ReinisSprogis]

Summary:
This update introduces customizable loading placeholders for the Tile widget, offering both a default grid and the option to use a custom widget as a placeholder. The feature is controlled by the showPlaceholder boolean parameter within TileLayer. When showPlaceholder is true, developers can either rely on the default grid or specify their own widget to be displayed during tile loading. This addition significantly improves the app's visual feedback during slow loading times.

Key Changes:

showPlaceholder Parameter: Determines whether any placeholder is shown during tile loading. A false value results in no placeholder, maintaining a clean but potentially empty tile space during load.
Custom Placeholder Widget: When showPlaceholder is true, and a custom widget is provided via the placeholder parameter, this widget will be displayed as the loading indicator. This allows for unique, app-specific loading visuals.
Default Grid Placeholder: In the absence of a custom widget (with showPlaceholder set to true), a default grid serves as the loading indicator, enhancing the user's awareness of the loading process without overwhelming the visual experience.

Benefits:
Improved User Experience: Offers immediate visual feedback during tile loading, enhancing the perception of responsiveness, especially in low bandwidth scenarios.
Flexibility and Customization: Developers can customize tile loading experience with custom placeholders.

[ReinisSprogis]

#1853 would make this invalid. Need to make changes if #1853 ended up approved.

[JaffaKetchup]

Closing this PR, as it looks as though we're going to progress with #1853.