Make it possible to use forceActiveFocus on TextField

I believe the manual forceActive focus is no longer necessary
since TextField is now a focus scope and it is enough to set the
focus property on the TextInput. Without this patch, calling
foceActiveFocus does not actually give the text cursor focus.

Change-Id: I2dda5c7c0e9511f0d8b6817dcd7b78230e6cb823
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>