* The crumbly floor doodad was made 50% larger.
* New doodad: Small Key and Small Key Door. These work like the colored
doors and locks except each Small Key is consumed when it unlocks a
door. The door's appearance is of iron bars.
* The inventory HUD displays a small quantity label in the lower-right
corner of items that have a quantity, such as the Small Key. This is
done as a Canvas.CornerLabel string attribute on uix.Canvas.
* The "give all keys" cheat adds 99 Small Keys to your inventory.
* 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.
* Added an inventory system for actors as a replacement to the arbitrary
key/value data store. Colored keys now add themselves to the player's
inventory, and colored doors check the inventory.
* Inventory is a map[string]int between doodad filenames
(red-key.doodad) and quantity (0 for key items/unlimited qty).
* API methods to add and remove inventory.
* Items HUD appears in Play Mode in lower-left corner showing doodad
sprites of all the items in the Player's inventory.
