This refactor combines the functions
gcal_utils_launch_online_accounts_panel and
gcal_utils_launch_datetime_settings_panel into a single function that
takes a panel ID as an argument.
Shortcut is added so that time-related system settings can be easily
accessed. See #1023
The code is largely just a copy-paste of the code that opens the
"Online Accounts…" menu.
Fixes#1023