Added bugfixes

Cherry-picked these bugfixes from upstream:

  * Fix 'GDK_IS_MONITOR (monitor)' failed message
    Partily fixes https://gitlab.com/rastersoft/desktop-icons-ng/-/issues/229
  * Keep the application if there are no connected monitors
    Completely fixes https://gitlab.com/rastersoft/desktop-icons-ng/-/issues/229
  * Ensure that DING isn’t launched twice
  * Fix bug https://launchpad.net/bugs/1976204 (hangs when using two monitors with
    different zoom settings)

LP: #1978330, #1978331, #1976204