* Increase the default window size from 800x600 to 1024x768.
* Move the drawing canvas in EditorMode to inside the EditorUI where it can
be better managed with the other widgets it shares the screen with.
* Slightly fix Frame packing bug (with East orientation) that was causing
right-aligned statusbar items to be partially cropped off-screen. Moved a
couple statusbar labels in EditorMode to the right.
* Add `Parent()` and `Adopt()` methods to widgets for when they're managed
by containers like the Frame.
* Add utility functions to UI toolkit for computing a widget's Absolute
Position and Absolute Rect, by crawling all parent widgets and summing
them up.
* Add `lib/debugging` package with useful stack tracing utilities.
* Add `make guitest` to launch the program into the GUI Test.
The command line flag is: `doodle -guitest`
* Console: add a `close` command which returns to the MainScene.
* Initialize the font cache directory (~/.cache/doodle/fonts) but don't
extract the fonts there yet.
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.
