Add style support for Adw.Banner

[ryonakano]

Problem

Gtk.InfoBar was deprecated in Gtk 4.10 and it seems like we can use Adw.Banner as a replacement but we don't seem to support its CSS name banner, resulting no styles applied:

Proposal

Add support for banner style

Prior Art (Optional)

No response

[ryonakano]

Adw.Banner does not seem to support style variants like info, warning, and error unlike Gtk.InfoBar, so it would be nice to cover that use case by creating our own replacement for Gtk.InfoBar in Granite instead of styling Adw.Banner.