Add an input method is composing property to TextEdit and TextInput.

When a text input is composing (has preedit text) it will forward mouse events to the input context instead of handling them itself. If a MouseArea is used over a text input however then the input will never receive the events and so it would be useful to know when the text input is composing so mouse areas can be disabled and other functionality which may interfere with the input context can be tempoarily disabled.