Finish "backend/render"

[quarnster]

This should contain a common code path for frontends to deal with functionality related to rendering the editor.

[quarnster]

This might have to be split into separate issues in the future. Right now it's just dealing with issues related to displaying a single View, but for example splitting a window into displaying multiple views is another item that might share much code between frontends.

[quarnster]

This is blocking all frontends btw, so deserves attention from others. All frontends should use the API defined here to query the backend for information on how to render a view (foreground and background colors, font styles, outlines, underlines and other info).

This code isn't currently used and is biased towards Sublime Text 3's current functionality. In other words, regions are marked with a scope name and then the colour is determined by looking up the scope in a textmate theme (syntax colouring config).

It would be more powerful to allow to bypass this deferred lookup and being able to specify colors and other rendering hints directly.

[quarnster]

Issue is too generic, lets upon up separate issue numbers as we run into issues where the current api is lacking