* Adds global modal support in the pkg/modal/ package. It has easy
Alert() and Confirm() methods to prompt the user before calling a
callback function on affirmative response.
* Modals have global app state: they're processed in the main loop in
pkg/doodle.go similar to the global command shell.
* When a modal is active, a semitransparent black frame covers the
screen (gameplay loop paused, last game frame rendered below) and the
modal window appears on top.
* The developer console retains higher priority than the modal system
and always renders on top.
* Editor Mode: track when the level pixels have been modified, and
confirm the user about unsaved changes when they attempt to close the
level (New, Open, Close, etc.)
* Global: the Escape key no longer immediately shuts down the game, but
will confirm the user's intent via a modal.
* File->Quit in the Editor Mode also invokes the confirm shutdown modal.
