* Fix the level collision bug that allowed clipping thru a ceiling while
climbing up a wall.
* Fix the scrolling behavior to keep the character on-screen no matter
how fast the character is moving, especially downwards.
* Increase player speed and gravity.
* New cheat: "ghost mode" disables clipping for the player character.
* Mark an actor as "grounded" if they fall and are stopped by the lower
level border, so they may jump again.
* Debug mode: no longer enables the DebugOverlay (F3) by default, but
does now insert the current FPS counter into the window title bar.
* ui.Frame: set a default "mostly transparent" BG color so the frame
background doesn't render as white.
* Add the MenuScene which will house the game's main menus.
* The "New Level" menu is first to be added.
* UI lets you pick Page Type and Wallpaper using radio buttons.
* Page Type: Unbounded, Bounded (default), No Negative Space, Bordered
* Fix bugs in uix.Canvas to fully support all these page types.
Fixes:
* Move the call to CollidesWithGrid() inside the Canvas instead of
outside in the PlayScene.movePlayer() so it can apply to all Actors
in motion.
* PlayScene.movePlayer() in turn just sets the player's Velocity so the
Canvas.Loop() can move the actor itself.
* When keeping the player inside the level boundaries: previously it was
assuming the player Position was relative to the window, and was
checking the WorldIndexAt and getting wrong results.
* Canvas scrolling (loopFollowActor): check that the actor is getting
close to the screen edge using the Viewport into the world, NOT the
screen-relative coordinates of the Canvas bounding boxes.
