Fix input field tab handling

When searching for the current field in the field list to find the
previous or next one, we check the field start and compare it with
the cursor position.
But with the new input fields, the cursor can actually be anywhere
in the field, so we actually have to search for the start position
of the input field at the cursor position.

Change-Id: I26526524eccfdbea41c6bf69a460fa64248f50ca
Reviewed-on: https://gerrit.libreoffice.org/10837
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>