Adds the first features to Edit Mode to support creation of Doodad
files! The "New Doodad" button pops up a prompt for a Doodad size
(default 100px) and configures the Canvas widget and makes a Doodad
struct instead of a Level to manage.
* Move the custom Canvas widget from `level.Canvas` to `uix.Canvas`
(the uix package is for our custom UI widgets now)
* Rename the `doodads.Doodad` interface (for runtime instances of
Doodads) to `doodads.Actor` and make `doodads.Doodad` describe the
file format and JSON schema instead.
* Rename the `EditLevel()` method to `EditDrawing()` and it inspects the
file extension to know whether to launch the Edit Mode for a Level or
for a Doodad drawing.
* Doodads can be edited by using the `-edit` CLI flag or using the
in-game file open features (including `edit` command of dev console).
* Add a `Scrollable` boolean to uix.Canvas to restrict the keyboard
being able to scroll the level, for editing Doodads which have a fixed
size.
* Add ui.Window to easily create reusable windows with titles.
* Add a palette window (panel) to the right edge of the Edit Mode.
* Has Radio Buttons listing the colors available in the palette.
* Add palette support to Edit Mode so when you draw pixels, they take
on the color and attributes of the currently selected Swatch in your
palette.
* Revise the on-disk format to better serialize the Palette object to
JSON.
* Break Play Mode: collision detection fails because the Grid key
elements are now full Pixel objects (which retain their Palette and
Swatch properties).
* The Grid will need to be re-worked to separate X,Y coordinates from
the Pixel metadata to just test "is something there, and what is
it?"
First pass at a level storage format to save and restore maps.
To save a map: press F12. It takes a screenshot PNG into the
screenshots/ folder and outputs a map JSON in the working directory.
To restore a map: "go run cmd/doodle/main.go map.json"
