Replace deprecated Gtk.ComboBox with Gtk.DropDown

[leonardo-lemos]

Closes #400
Closes #425

This PR introduces the following improvements:

• Each dropdown (Resolution, Rotation, Refresh Rate, and Scale) is now implemented as a separate custom widget, with its own logic and signals.
• Business logic and UI for these dropdowns have been moved out of DisplayWidget and VirtualMonitor, resulting in a more modular and maintainable architecture.
• The DisplayWidget constructor has been significantly simplified, focusing only on creating and connecting the new widgets.
• Interactions between widgets now use signals to decouple UI components from business logic.
• Deprecated Gtk.ComboBox and related data models have been fully replaced by the new dropdown widgets.
• FiltersView now uses Variant.maybe for more robust null value handling.
• Because Gtk.DropDown does not support tree hierarchies, all available resolutions are now presented in a single flat list in the resolutions dropdown.
• Some Meson deprecation warnings have been resolved.

[leonardo-lemos]

Needs more testing on changing scale.