Ability to customize window tile previews

[AdrianVovk]

When snapping/tiling windows to the edges of the screen, a little preview animation plays to show what the final size of the window would be. This preview view has all of its visual elements hard-coded into it. Perhaps some customization options could be exposed?

• Color: the most important. I'm currently modding Wayfire at build time to make these colors fit in with the rest of my desktop environment
• Animation: I'd like to make these do the existing zoom animation on open, and then fade out on close