We subscribe to Sway's 'input' events, and use these to expose input
devices' active XKB layout.
The module is configured by specifying a list of 'identifiers'; these
are the input devices (keyboards, typically), that we'll be
monitoring. All other input devices are ignored.
'content' is a template, and the module will instantiate a dynlist
with a 'content' for each *existing* input found in the 'identifiers'
list.
We also monitor for device 'added' and 'removed' events, and update
our internal list of existing inputs.
This means the user can configure a set of identifiers, and only those
that are actually present will be displayed. If a device that is
listed in the 'identifiers' list is added, it will be displayed. If it
is removed, it will no longer be displayed.
