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.
The main advantage is when targeting sway/wayland instead of i3/x11;
sway doesn't package ipc.h. There *is* an ipc.h in the sway sources,
but it doesn't define e.g the header format, and has different names
for the constants.
