A window can now say that a text field has the focus and where its caret
is — Window::SetTextInput(active, caret rectangle in physical pixels,
purpose) — which is what opens the on-screen keyboard on a phone (Maliit
under KWin, squeekboard under phoc). What the input method produces comes
back as events: committed text as onTextInput, the composing text as
onPreedit (empty clears it), deletions around the cursor as
onTextInputDelete; done() applies them in the protocol's order. The state
a window wants is remembered and sent again whenever the keyboard focus
returns to it. The protocol's client code is generated with wayland-scanner
1.26.0 from wayland-protocols' text-input-unstable-v3.xml, under lib/ like
the others.
