Changes in 1.15.0

• Switch to the Meson build system, and drop Autotools
• Drop most use of private GNOME API; the following portal interfaces
  have been removed:
  • org.freedesktop.impl.portal.Screenshot
  • org.freedesktop.impl.portal.ScreenCast
  • org.freedesktop.impl.portal.RemoteDesktop
  • org.freedesktop.impl.portal.Background
• Do not add duplicate notifications
• Expose the org.gnome.desktop.calendar settings
• Fix type for org.gnome.desktop.background.picture-uri setting
• Support current_folder in org.freedesktop.impl.portal.FileChooser
• Update list of translatable files
• Translation updates