Add Color & Roughness painting

[TokisanGames]

• Adds paintable color (RGB on colormap). Paint White to erase.

• Adds paintable roughness, (A on colormap). Offers a -100% to +100% slider, which translates into a -1 to +1 modifier on the textures roughness. Painting 0% means revert to texture roughness. 100% makes rougher. -100% makes glossy, which makes textures look wet. Add some dark grey on color along with reduced roughness to complete the look.

• Consolidates the row of buttons into a panel, and right-justifies Advanced button

• Adds pickers to pick up color, roughness, or height values from the terrain maps for the respective tools

• Separates toolbar loaders from Editor.gd into its own UI class.

• Hides Slope option until implemented. See Implement Slope Sculpting Tool  #96.

Fixes #42, Fixes #25