pdCanvas: rework handling of "auto-activate layer beneath mouse"

Big change here.  This feature hasn't seemed quite right up to this
point, but I think I've finally nailed down the most intuitive and
accurate implementation.

If "auto-activate layer beneath mouse" is selected, the layer under the
mouse is no longer auto-activated on MouseMove.  Instead, a message in
the status bar displays which layer will receive focus if the mouse is
clicked at that point.  This is unobtrusive, but still provides feedback
on what the user's next action will affect.

Similarly, the cursor is now properly updated against the mouse
position, including factoring in when the mouse is over a transparent
region of a layer and "ignore transparent pixels" is set.  (I have also
re-enabled that option on the interface.)  This way, the user will
reliably be given a Move cursor if an action will actually move the
active layer.  If a Move cursor is not displayed, the user knows that
clicking that point will cause a new layer to activate.

Also, layer transform nodes are now given preference during mouse
events.  Thus, if the mouse is not over the active layer, but it *is*
over one of the active layer's POIs, the POI will still be interacted
with instead of a new layer.  This allows the user to do something like
resize a layer, even if it lies beneath other layers.

Lots of odd and complex interactions for this feature, with changes
having to be handled differently by MouseMove and MouseDown, and with
those changes then affecting everything from the cursor to the layer
stack to the on-screen display.  But I think everything plays together
nicely now, and it does so much more elegantly than the same feature in
say GIMP.

Still to-do is custom cursors apart from the system cursor, which would
make this feature even better.  (e.g. an Arrow cursor with a small
resize or move modifier in the corner.)  I don't like the system cursors
because it's not clear where the cursor hotpoint is.

However, an overhaul of custom cursors is a big job that won't be
handled until I move the program to standalone theme files, which is
still a ways out...