feat: Port to libadwaita

[ryonakano]

Fixes #147

Changes Summary

• README.md
  • Update dependency list
• com.github.ryonakano.konbucase.yml
  • Enable granite build option because this file used to build the app for elementary OS
• data/resources/ui/main-window.blp
  • Port to libadwaita
• meson_options.txt
  • Add a build option for elementary OS builds
• src/Application.vala
  • Subclass Adw.Application
  • Use Adw.Application.style_manager instead of our custom StyleManager which requires Granite
  • Allow using elementary stylesheet on Pantheon
• src/Define.vala
  • Salvage string definitions from StyleManager.vala which still necessary
• src/StyleManager.vala
  • Remove in favor of Adw.Application.style_manager
• src/Util.vala
  • Update conversion methods between Action.state and Adw.ColorScheme
  • Add conversion methods between string representation of GLib.Settings values and Adw.ColorScheme
• src/View/MainWindow.vala
  • Port to libadwaita
• src/meson.build
  • Add libgranite to the dependencies
  • Make Granite optional dependency