Improve Editor a bit

- Add Image Preview in "Add Image Dialog"
- Add Texture Preview & Size Selector in Layers List
- Change points colors to Blue of Selected Quad
- Highligth quad when move points.
- Add Menus "View" & "Brush Mode"
- Add Addition mode to Brush (Omit index 0 in BrushDraw)
- Shortcut (CTRL+Q) to add a Quad in Mouse Position
- Other stuff...