gui: don't mix up display value with actual value

In the case of password entry, the two differ. Password entry was broken
by the earlier commit entitled "Improve input box text handling" because
it started using the displayValue as the actual value, even in cases where
they differ because of a mask value.

Change-Id: Iaf5a67e1d928f34595962a1f1b80eebb64e8b493