[geary] (20 commits) ...Merge branch 'mjog/mutiple-main-windows' into 'mainline'

[Michael Gratton <mjog src gnome org>]

Summary of changes:

  419bc6d... Remove GearyApplication.instance global var (*)
  5687636... Rename GearyApplication class and source to match code styl (*)
  239fdce... Move MainWindow to Application package (*)
  0dce504... Promote AccountContext to a top-level internal class (*)
  e2a3902... Drop Application.Controller access to internal (*)
  ab8755a... Add Application.Controller::account_available and unavailab (*)
  3ca121f... Move Application.MainWindow handling from Controller to Cli (*)
  3f6fe1f... Clean up Application.Controller and MainWindow shutdown (*)
  f4f9ff4... Improve logging initialisation and startup (*)
  0eaa3ac... Rework how Application.Client manages application holds (*)
  8f8709f... Add support for opening additional main windows (*)
  1ea97dc... Move global GTK CSS loading from main window to the app (*)
  446e9b8... Always update Application.MainWindow title (*)
  ccb1135... Show HeaderBar close buttons by default (*)
  af2b799... Select account/folder/conversations in new windows (*)
  68aaec3... Open conversations in new window on activate (*)
  a760e26... Improve UX when automating folder/conversation selection (*)
  fbc47a1... Ensure the inbox of the first account is selected on app st (*)
  51f0efe... Remove Application.Controller::display_main_window_if_ready (*)
  fbb6f8d... Merge branch 'mjog/mutiple-main-windows' into 'mainline'

(*) This commit already existed in another branch; no separate mail sent