Move tool: allow for auto-activation of layer beneath the mouse

I've gone through multiple iterations of this feature, and I think this
implementation is the best compromise.

One of my frustrations with GIMP is that you have no idea of what layer
the mouse will affect until you actually click and start moving an
object.  When working with any layer that has thin boundaries between
opaque elements - such as text - you'll often end up picking a layer
other than the one you wanted, and it can take several tries to select
the right one.

I solve this in PD by completely activating the layer under the mouse
during the mouse_move event.  This way, you see the targeted layer's
outline (and resize nodes) if those are enabled, and the layer will also
be highlighted in the layer toolbox.  This way there's never a
possibility of "surprise" layer activations, as you'll always know in
advance which layer will be affected by a mouse action.

I've also implemented an option for ignoring transparent areas of layers
when auto-activating the layer beneath the mouse, but I've hidden the
option for now because I don't actually like it.  Resizing a layer while
this option is active is nearly impossible, if the layer has any kind of
transparency around its edges (e.g. a logo surrounded by dead space).
When the mouse moves over the dead space, the layer *beneath* that
current one gets selected.  However, if the mouse moves over the top
layer's resize nodes - located at its corners - the layer will magically
re-activate, despite the mouse being over a transparent region.

There's no good way to handle this, at least not one I can think up,
short of disabling layer resizing while in "ignore transparent pixels
when auto-selecting a layer" mode.  I don't like that either, so I'm
simply hiding the option until I can think up a better solution.