* 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.
* Discovered a bug where if you hit the Undo key to erase pixels and an
entire chunk became empty by it, the chunk would have rendering errors
and show as a solid black square instead of the level wallpaper
showing through.
* Chunks that have no pixels in them are culled from the chunker
immediately when you call a Delete() operation.
* The level file saver also calls a maintenance function to prune all
empty chunks upon saving the file. So existing levels with broken
chunks need only be re-saved to fix them.
* Add a Level Exit doodad, which for now is a little blue flag on a pole
that reads "END"
* JavaScript API: global function EndLevel() will end the level. The
exit doodad calls this when touched by the player.
* Add a "Level Completed" alert box UI to PlayScene with dynamic button
layouts.
* The alert box pops up when a doodad calls EndLevel() and contains
action buttons what to do next.
* "Play Again" restarts the current level again.
* "Edit Level" if you came from the EditorScene; otherwise this button
is not visible.
* "Next Level" is a to-be-implemented button to advance in the single
player story mode. Only shows up when PlayScene.HasNext=true.
* "Exit to Menu" is always visible and closes out to the MainScene.
