Until gi-docgen is declared stable support either of them.
This effectively means that hand builds and CI builds will use
gi-docgen, but distro builds use gtk-doc-tools.
This allows plugins to set the battery power state of the *machine* which means
we can automatically inhibit devices with FWUPD_DEVICE_FLAG_REQUIRE_AC set.
It also allows to set the BatteryThreshold to 25% for Lenovo hardware, and we
can override other vendors with further quirks as required.
Fixes https://github.com/fwupd/fwupd/issues/3084
There is a lot of code in fwupd that just assigns a shared object type to
a FuPlugin, and then for each device on that plugin assigns that same shared
object to each FuDevice.
Rather than proxy several kinds of information stores over two different levels
of abstraction create a 'context' which contains the shared *system* state
between the daemon, the plugins and the daemon.
This will allow us to hold other per-machine state in the future, for instance
the system battery level or AC state.
