Introduce DiaRenderer::draw_rotated_image() and move some code to helper
functions for reuse. Implement DiaCairoRenderer::draw_rotated_image() for
testing the new functionality (mainly with transform-variations.svg).

Give "Standard - Image" an angle property: copy, load, save it.
Set Image::angle in image_transform().

Unrelated: use element_update_connections_rectangle() to ensure proper
directions initialization with less code.

Open issues:
 - for now only the cairo renderer implements draw_image_rotated()
 - either the angle should be limited like for "Standard - Box" or the
   connection point directions for angles outside [-45...45deg] must be fixed