Add TextField/Area::inputMethodComposing

In order for the Android style to be able to control the visibility of
the text cursor handle as appropriate, it needs to know when the input
method is composing ie. when the user is typing.

Change-Id: If8e3ac66b63c938659a5d9044d75eafd348cd6a3
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>
TextField/AreaStyle: add styleData.activated() signal for handles

This makes it possible for the Android style to implement "transient"
handles that fade out when idle, and show up when the editor is clicked

Change-Id: I53da6a4d5345c98b8b7286b30006982eaf17df45
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>