theme: Adds very basic theme support.

[abaire]

Puts in the foundation for theme selection.

• Modifies Makefile to blindly copy anything under Resources/NeXThemes and drops the new_all workaround by adding RESOURCES to TARGET.
• Adds a Theme JSON parsing class.
• Moves the default theme resources into a default theme subdir and adds a .json definition file for them.

Some nice followups would be:

1. Extend Font so the font color can be changed.
2. Add a menu item to list and select themes.
3. Add a launcher image so it's clear that NeX hasn't frozen when the user launches a new XBE (we can flash this up and with the current nxdk it should be automatically persisted until the newly launched XBE takes over, though XboxDev/nxdk#507 will need to be fixed)

[abaire]

@kosmas12 I took a stab at putting in a simple foundation that uses JSON files to define themes. Assuming @dracc is OK with this approach, maybe you'd like to rebase your PR and take on the theme directory scanning and menu selection parts?