Add the ability for the free version of the game to allow loading levels that
use embedded custom doodads if those levels are signed.
* Uses the same signing keys as the JWT token for license registrations.
* Levels and Levelpacks can both be signed. So individual levels with embedded
doodads can work in free versions of the game.
* Levelpacks now support embedded doodads properly: the individual levels in
the pack don't need to embed a custom doodad, but if the doodad exists in
the levelpack's doodads/ folder it will load from there instead - for full
versions of the game OR when the levelpack is signed.
Signatures are computed by getting a listing of embedded assets inside the
zipfile (the assets/ folder in levels, and the doodads/ + levels/ folders
in levelpacks). Thus for individual signed levels, the level geometry and
metadata may be changed without breaking the signature but if custom doodads
are changed the signature will break.
The doodle-admin command adds subcommands to `sign-level` and `verify-level`
to manage signatures on levels and levelpacks.
When using the `doodad levelpack create` command, any custom doodads the
levels mention that are found in your profile directory get embedded into
the zipfile by default (with --doodads custom).
* Free (shareware) versions of the game will not be able to Publish
Levels (attach custom doodads to the level file) and they will not be
able to load a level which relies on embedded doodads.
* The UI for the Publish Level window is still available, but clicking
on the confirm button will just open the Register (License) window.
* When loading a level containing embedded doodads: if some can't load
because they're embedded and you're using the free version of the
game, the error message is customized to reflect that.
* New command-line tool: doodle-admin for signing license keys for
users. Includes functions to initialize a keypair, sign license keys
and validate existing keys.
* The Main Menu screen shows a blue "Register Game" button in the bottom
right corner of the screen, for unregistered users only.
* In Edit Mode, there is a "Help -> Register" menu item that opens the
License Window.
* The License UI Window lets the user select the license.key file to
register the game with. If registered, a copy of the key is placed in
Doodle's profile directory and the licensee name/email is shown in the
License UI window.
* Unregistered games will show the word "(shareware)" next to the title
screen version number and Edit Mode status bar.
* No restrictions are yet placed on free versions of the game.
