Layer names can now be edited by double-clicking

Still some problems to solve here.  VB does incredibly weird stuff with
focus handling.  For reasons I can't solve, if I click a different tool
button while editing a layer name, the layer name text box will never
again receive an Enter keypress event.  (????)  It may have something to
do with clicking another form, but if that's the case, re-clicking the
text box should restore focus.  I can type other characters just fine,
and all their keycodes arrive as expected, but the Enter key does not.