Always implement text input "canPaste" property

Always implement text input "canPaste" property

This is needed for TextField control for instance.
If QT_NO_CLIPBOARD is defined, canPaste will always return false.
Fixes dialogs auto test when QT_NO_CLIPBOARD is defined.

[ChangeLog][QtQuick][Platform Specific Changes] Added canPaste
property to TextInput element also on platforms that don't support
a clipboard (QT_NO_CLIPBOARD is defined).

Change-Id: Ia85205672ba59c1fce70852458c514b03d332de6
(cherry picked from commit c2d802049e48d1d6f0bce6eb0972fb26dc4d6ce3)
Reviewed-by: Fabian Bumberger <fbumberger@rim.com>
Reviewed-by: Bernd Weimer <bweimer@blackberry.com>