Alternate attempt for backend interface.
As far as I got with porting to the backend.
drawTextsR now calls drawTextR if only a single line text has to be rendered.
Removed bDrawText and bDrawTextR from the interface, since bDrawTextsR is the most general.
Unified the three drawText functions in the backend interface.

Merge branch 'abstract-drawing' into backend-experiment

Conflicts:
	chart/Graphics/Rendering/Chart/Types.hs
Removed accidental commits.
Added the old interface functions defined in terms of the new interface.
Pushing the generalisation further.
Pushed the generalisation through!
Added export of custom toRenderable function.
Export Backend.Cairo module for the time being.
Fixed some compilation issues of the type system (without a workaround).
Removed misleading comments.
Simplified the problematic layout code.
Added reference SVG and PNG files to compare with the output of the new code.

HTML files to compare new and reference side by side.
Removed ToRenderable completely, because all of its usage was ambigious and needed separate type annotations.
Updated export list with ...ToRenderable functions needed for the tests.
Update of the export list with another ...ToRenderable.
Changed signature of layout1ToRenderable and layout1DddToRenderable. All tests seem to expect this interface.
Modified the tests to work with the new code.