New class SolidFill in order to fill image with monochromatic background

New class SolidFill in order to fill image with monochromatic background.
It allows one to write image constructors in a consistent syntax
for both gradient/hatched and for solid backgrounds instead of two
completely different syntaxes:

```ruby
Image.new(size_x, size_y, HatchFill.new('white', 'red'))
Image.new(size_x, size_y, SolidFill.new('bisque'))
```
vs

```ruby
Image.new(size_x,size_y, HatchFill.new('white','red'))
Image.new(size_x,size_y){ self.background_color = 'bisque' }
```