Port to GTK 4

Two things were visually modified

• The .compressed-entries-label background color was set to accent_bg_color (blue), since borders now is invisible in light mode and has bad contrast on dark mode
• Back button on the search popup is now a GtkButton, ModelButton is a private widget, and the inverted property is no available anymore making it look weird, this could be fixed to look as it was It was styled following menu model's sub menu back button

See Initiatives#26