Added support for multiple download sources

[CriticalFlaw]

Changelog:

• Added support for multiple download paths. In the schema file, the new "Download" group can contain multiple sources, each with a display name and download link. The list will then be displayed on the button tray at the bottom of the screen.
• Updated the button tray UI to fit the download source dropdown.
• Updated schema files to adopt the new feature. Hopefully without breaking for current users.

Notes:

• The "Update" field can be removed from schema files after this feature is released and enough users migrate to 2.5.
• The UI change to fit the new download source dropdown could use another pass. It doesn't look too great at the moment.