* Player character now experiences acceleration and friction when
walking around the map!
* Actor position and movement had to be converted from int's
(render.Point) to float64's to support fine-grained acceleration
steps.
* Added "physics" package and physics.Vector to be a float64 counterpart
for render.Point. Vector is used for uix.Actor.Position() for the sake
of movement math. Vector is flattened back to a render.Point for
collision purposes, since the levels and hitboxes are pixel-bound.
* Refactor the uix.Actor to no longer extend the doodads.Drawing (so it
can have a Position that's a Vector instead of a Point). This broke
some code that expected `.Doodad` to directly reference the
Drawing.Doodad: now you had to refer to it as `a.Drawing.Doodad` which
was ugly. Added convenience method .Doodad() for a shortcut.
* Moved functions like GetBoundingRect() from doodads package to
collision, where it uses its own slimmer Actor interface for just the
relevant methods it needs.
* Add support for the LineTool and RectTool while in the EditorMode to
easily draw straight lines and rectangle outlines.
* Key bindings were added to toggle tools in lieu of a proper UI to
select the tool from a toolbar.
* "F" for Pencil (Freehand) Tool (since "P" is for "Playtest")
* "L" for Line Tool
* "R" for Rectangle Tool
* Load SDL2 fonts from go-bindata storage so we don't have to ship
external font files on disk.
* Dedupe names of doodads so we don't show double on the front-end
(go-bindata bundled doodads + those on local filesystem)
* Use go-bindata for accessing wallpaper images.
* Better flashed messages walking you through the Link Tool.
* Stylize the title screen (MainScene) by rendering a live example level
as the background wallpaper, with mobile doodads in motion.
* On the Doodads tab is the Link button to enter the Link Tool.
* Click Link, then click the 1st doodad on the level, then click the 2nd
doodad to complete the link.
* The actors struct in the Level holds the link IDs for each actor.
