pdGraphicsPath: new class for managing path data

This is basically just a thin wrapper to the GDI+ GraphicsPath object,
but it makes path management waaay easier.  It also allows me to do neat
things like unify a ton of old, scattered UI code.

The first beneficiary of the change is selection tools.
Selection tools: make Outline mode available for all tools

...except Magic Wand, because it's a lot more complicated.  Maybe
someday.

I'd like to enable a true outline for bordered selection types (instead
of just showing the selection shape), but this has some nasty
complexities on polygon and lasso tools.  Might revisit in the future.
Window manager: give the tool options toolbox positioning precedence

This makes the main window symmetrical, and also frees up horizontal
room for tool options (which we'll need for things like the text tool).
UI: start work on overhauling toolboxes

This is going to be an ongoing project throughout 6.6, and I can't
complete all of it until I break down and finally implement a
PD-specific button user control, but that doesn't mean I can't start
working through parts of it.
pdVoronoi: improve comments
pdDictionary: add Variant-specific function

Helpful for testing equality on raw properties
pdGraphicsPath: add rounded rectangle function

Also, self-validate the requested radius, and update the GDI_Plus module
equivalent to match
Some UCs: add toolbox icons

It makes me happy when these look pretty in the toolbox.  :p
Start toolbox overhaul on main window

Feedback is of course welcome, but right now everything should be
treated as if it's just a placeholder.  Icons and buttons in particular
are going to be reworked once I have a new PD-specific button UC ready.
(JCButton is going away.)  I'm also going to rework the top of the
toolbox to include some new buttons, so ignore the odd layout at
present.

In the meantime, I have started the incredibly unpleasant work of
rewriting the back-end that controls all selection tool UI.  I was able
to simplify a lot of existing UI in this commit, and of course fix a
number of bugs while I was at it.  The new tools should be coming
together pretty nicely now, with only some minor issues left to work
out.