Rewrite backend with GObject / widget subclassing

This MR implements several new GObject based types. They're replacing the Rust messaging passing approach.

GTK Widgets

• SoukPackageRow
  • package
• SoukPackageTile
  • package

Transaction

• SoukTransaction
  • package
  • action
  • state
• SoukTransactionMode [enum]
• SoukTransactionState
  • message
  • percentage
  • mode

Package

• SoukPackage
  • kind
  • name
  • arch
  • branch
  • remote
  • remote_info
  • installed_info
  • transaction_action
  • transaction_state
• SoukPackageAction [enum]
• SoukPackageKind [enum]
• SoukInstalledInfo
  • commit
  • installed_size
  • deploy_dir
• SoukRemoteInfo
  • commit
  • installed_size
  • download_size

Flatpak Backend

• SoukFlatpakBackend
  • installed_packages
  • new_transaction [signal]

There are still few open tasks, but I think it's better to implement them in a follow-up MR, since this MR would be get too big otherwise.

• The search / listview perfomancy is poor. This needs benchmarking Fixed! (Of course there's always room for further improvements ^^)
• There's a memory leak somewhere. Also probably in the search code. Fixed!
• SoukFlatpakBackend::reload_installed_packages() takes a long time, and is the reason of the high startup times of Souk