Add capabilities to QPlatformInputContext.

Add a capability enumeration to QPlatformInputContext and
use that to turn off input methods for hidden text depending
on platform support. Disable on Windows.

Task-number: QTBUG-40691
Change-Id: I9909005de1f21316ec8f64e2729f1fffcd37c7c3
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Björn Breitmeyer <bjoern.breitmeyer@kdab.com>
Reviewed-by: Liang Qi <liang.qi@theqtcompany.com>