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 Graphical corruption when using HalQuickRebootRoutine to return to dashboard 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?