Unfortunately selecting the field in SwEditWin::MouseButtonDown()
prevents the click field handling code in SwEditWin::MouseButtonUp()
from running.  Instead select the field in MouseButtonUp and
SwWrtShell::ClickToField(), which seems to work better.
(regression from 94721b2a)

Change-Id: I3d769487b9d5225ffd07b384fbb0ce01e0733be5