This adds the scaffolding for an internal plugin architecture. No such
plugin currently exists and any plugin implemented against this
architecture merely is plugin-like in the event flow, not actually
external to libinput.
The goal of this architecture is to have more segmented processing
of the event stream from devices to modify that stream before libinput
ever sees it. Right now libinput looks at e.g. a tablet packet and then
determines whether the tool was correctly in proximity, etc.
With this architecture we can have a plugin that modifies the event
stream that the tool is *always* correctly in proximity and the tablet
backend itself merely needs to handle the correct case.
The event flow will thus logically change from:
evdev device -> backend dispatch
to
evdev device -> plugin1 -> plugin2 -> backend dispatch
The plugin API is more expansive than we will use immediately, it is the
result of several different implementation branches that all require
different functionality.
Part-of: <https://gitlab.freedesktop.org/libinput/libinput/-/merge_requests/1217>
