UI textures regrouped under Resources/

[kvakvs]

What does

This is the first step in transition to the new skin for the main menu.
The future look was proposed and discussed in #523

• Groups textures under Resources into subdirectories
• Renames texture static storage classes and fields to contain less Texture or Texture2D

Quality Assurance

In my defense, i've played for 2 hours and didn't notice missing textures or anything misplaced.

[originalfoo]

LGTM 👍