Size and Rotation can be defined by an expression for all symbols
composing a marker. Width can be defined by an expression for all
symbols composing a line.
For markers, a legend is generated for varying sizes. This allows
multivariate analysis legend in the case of classified/graduated colors.
The offset is now set along with size to maintain the relative position
of symbols composing a marker.
An asistant, with preview, is accessible through the data defined button
to help the user define the size expression. Three methods are
available: Frannery, Area and Radius.
Added a widget for use in categorized/classified symbology gui to set
the expression if needed. The assistant is also available from it.
