Media queries types

[Mehdi]

Mixins for media queries types are already written, but not documented outside their sources yet.

Maybe some custom media queries could/should contain a type (print, screen, speech) by default.

• Document --print, --screen and --speech.
• Drop usage of only and all keywords as they are legacy. If someone request them, think about adding some legacy mode.
• Evaluate which custom media queries could receive a type by default.