Let the user choose Bluetooth status on login

Currently there is no way for the user to choose whether Bluetooth is on
or off at login. Instead, it is decided automatically for them by the
presence or absence of the `AutoEnable=true` key in /etc/bluetooth/main.conf.
This is a root-owned file that does not permit user configuration if it
is shipped by a distro, because it will be blown away on the next system
update.

This commit adds a GUI to allow the user to determine the Bluetooth
status on login: always enabled, always disabled, or remember the
previous status. The default is to remember the previous status, with
the initial status on first boot being determined by the distro's
default value in the /etc/bluetooth/main.conf config file if present; if
not present, bluez's own default value is "disabled". Regardless,
thereafter the enablement status on boot will be determined by Bluedevil.

Implementation-wise, this is accomplished by adding support in the KDED
module for reading a new config file key set by the KCM which controls
what do do with Bluetooth when launched. It already has support for
saving and restoring status when the system goes to sleep and wakes up;
this was extended for shutdown/startup events too.

BUG: 440493
FIXED-IN: 5.23