[cleanup] remove stale prototypes
[transform] text rotation for "Standard - Text"

Rotations is done around the text handle position. This position is
only known to the object implementation. There is new render API
draw_rotated_text() taking the rotation center an angle, which is
much less invasive than making the underlying Text object aware of
angle and vertical alignment (and than not using it almost anywhere).
It is assumed that free text rotation will only be directly supported
by very few objects, so the burden should not be propagated to all
objects with text.
The initial version is should be working for almost all renderers
due to the default implementation of DiaRenderer::draw_rotated_text()
converting the text to path and rotating that.
[transform] svg: import transformation from <text transform="...">

With the last commit implementing "Standard - Text"::transform() text
transformation is much easier and more correct.
svg: fix text getting visible background during import

Could be seen with transform-variations.svg and default black/white
foreground/background settings.
svg: line-width was too small with rotated matrix

use transform_length() rather than only DiaMatrix::xx.
[transform] svg: export rotated text as text, rather than path

Implement SvgRenderer::draw_rotated_text() to have it only for SVG export,
but not for shapes. SVG round-trip works this way.