Layer visibility can now be toggled

Still working through the implications of various UI approaches.  One of
my goals is to try to keep the layer box (and viewport) as uncluttered
as possible, while still providing plenty of visual cues about layer
state.  To that end, the layer box is unlikely to end up a direct clone
of Photoshop/GIMP/etc.

One thing I would like to avoid is the need for a context menu on the
layer box (except for rare items, potentially).  I find it preferable to
provide frequent layer options directly on the layer box when a layer is
hovered: merge up/down, toggle visibility, delete, etc.  This is my
current project, as you can see from the visibility toggle.

That said, all hover interactions obviously present a problem for touch
users - something I'm aware of, and will revisit once I make a final
decision on the standard kb+m interactions.
Add dynamic tooltips to layer box

Also, more layer box UI tweaks.  I'll be testing lots of UI behavior
variations in the coming weeks - input welcome!