Add text-input support

[emersion]

This allows virtual keyboards and IME to work.

https://github.com/wayland-project/wayland-protocols/blob/master/unstable/text-input/text-input-unstable-v3.xml

[kchibisov]

I'm not entirely sure if there's a need implementing helpers for this protocol, knowing that it should be handled similar to how touch/pointer/keyboard are being handled in seat listener that user will create anyway https://github.com/Smithay/client-toolkit/blob/dceca4d13161cee577550742a1c1e4963600a0f8/examples/kbd_input.rs#L102.

@vberger if you have an idea what helpers could be added for it, I'll try to implement them in sctk, though.

[elinorbgr]

Yes, the process for retrieving the text_input for a seat is pretty similar to all other protocols that plug onto the seat list.

But the text_input interface is complex and requires a nontrivial amount of state tracking, so I think it'd definitely gain by having some helpers designed for it. I haven't really given much time thinking about how to design a good API for these helpers though.

[kchibisov]

But the text_input interface is complex and requires a nontrivial amount of state tracking, so I think it'd definitely gain by having some helpers designed for it. I haven't really given much time thinking about how to design a good API for these helpers though.

I'll start plumbing it into winit and will see how ti goes, since we have alacritty to test such things on. Then we may just move those bits into sctk, I guess.