* Adds pkg/savegame to store user progress thru Level Packs.
* The savegame.json is mildly tamper resistant by including a checksum
along with the JSON body.
* The checksum combines the JSON string + an app secret (in savegame.go)
+ user specific entropy (stored in their settings.json). If the user
modifies their save file and the checksum becomes invalid the game
will not load the save file, acting like it didn't exist, resetting
all their high scores.
Updates to the Story Mode window:
* On the LevelPacks list: shows e.g. "[completed 0 of 3 levels]" showing
a user's progress thru the level pack.
* Below the levels on the Detail screen:
* Shows an indicator whether the level is completed or not.
* Shows high scores (fastest times beating the level)
* Shows a padlock icon if levels are locked and the player hasn't
reached them yet. Pops up an Alert modal if a locked level is
clicked on.
Scoring is based around your fastest time elapsed to finish the level.
* Perfect Time (gold coin): player has not died during the level.
* Best Time (silver coin): player has continued from a checkpoint.
In-game an elapsed timer is shown in the top left corner along with the
gold or silver coin indicating if your run has been Perfect.
If the user enters any Cheat Codes during gameplay they are not eligible
to win a high score, but the level will still be marked as completed.
The icon next to the in-game timer disappears when a cheat code has been
entered.
* New Doodad: Checkpoint Flag. They update the player's spawn point
whenever the player passes one. The most recently activated
checkpoint is rendered brighter than the others.
* End Level Modal: the fake alert box window drawn by the Play Mode
is replaced with a fancy modal widget (similar to Alert and Confirm).
It handles level victory or failure conditions and can show or hide
all the buttons as needed.
* Gameplay: There is a "Retry from Checkpoint" option added, which
appears in the level failure modal. It will teleport you back to
the Start Flag or the last Checkpoint Flag you had touched, without
resetting the level -- your keys, unlocked doors, etc. will be
preserved so you can retry.
* Set a maximum speed on the "Camera Follows Actor" logic of 64
pixels per tick. This results in a smoother scrolling transition
when the player jumps to a new location on the map, such as by
a Warp Door.
* Update the default color palettes:
* All: Add a "hint" magenta color.
* Colored Pencil: Add a "darkstone" solid color.
Updates to the Doodads JavaScript API:
* SetCheckpoint(Point(x, y)): set the player character's spawn
position. Giving it Self.Position() is an easy way to set the
player spawn to your doodad's location.
