Move all Doodle source code into the src/ subpackage and move the
publicly shareable stuff into lib/, for example lib/ui and lib/render.
This cleans up the git root and helps make the Doodle UI library more
easily publishable as a separate open source project. Currently both
lib/ui and lib/render import one or two things from doodle/src that need
to be broken apart.
* 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.
