Add heartbeat to check if backend is alive.

Send a 'ping' request every 2 secs to ensure that the backend is
running.
Use polling instead of recv on the backend_proxy. This was already
implemented for the signaler.
Kill backend on quit if it does not respond.
Restrict access to the zmq certificates folder.
Set the standalone value for BaseConfig.
Merge remote-tracking branch 'refs/remotes/ivan/bug/set-standalone-flags-for-baseconfig' into develop
Merge remote-tracking branch 'refs/remotes/ivan/feature/restrict-certificates-permissions' into develop
Don't call the backend if it's not online.
Merge remote-tracking branch 'refs/remotes/ivan/feature/add-backend-alive-check' into develop
Add frontend alive check to the backend.

Stop the backend if the frontend process does not exist any more and
backend is not a daemon.
Merge remote-tracking branch 'refs/remotes/ivan/feature/add-frontend-alive-check' into develop
Improve hide and show main window on Ubuntu
Merge remote-tracking branch 'chiiph/bug/show_hide_improved' into develop
Disable user/pass remembering temporarily.

Set the keyring to None in order to simulate an always unavailable
keyring, that way we avoid the possibility of the user running into the
existing keyring issues. See https://leap.se/code/issues/4190

Update comparisons to do a proper comparison with `None`.
Fix login widget 'enabled' changer in order to change the 'remember'
widget *only* if we have an usable keyring.
Simplify and sort OS flags.
Add Linux autostart. Closes #4989.
Merge remote-tracking branch 'refs/remotes/ivan/bug/disable-remembering' into develop
Merge remote-tracking branch 'refs/remotes/ivan/feature/autostart-on-linux' into develop
Fix misused import.
Merge remote-tracking branch 'refs/remotes/ivan/bug/fix-import-error' into develop
Create the autostart path in case that does not exist.