Moved Display="none" hack & Added .Render check for : if (!Visible ||
!Displayable)

Also, minor change to logic of SvgBoolConverter so bad visibility values
(other than "hidden" or "collapse") wouldn't accidentally hide element

- exponent support in units (IE sucks!)
- inherit font-size and font-family from root element on text elements
- allow empty text elements if there is a non empty TSPan

Some programs (like InkScape) use single quotes for the font-family.
I've added the single quote character, to the trim.

Added support for dx and dy properties for text. A bit of plumbing was already in place so I completed that.
http://www.w3.org/TR/SVG11/text.html#TextElementDXAttribute
http://www.w3.org/TR/SVG11/text.html#TextElementDYAttribute

To get the Bounds, the Union method of the rectangle must be called only when the first rectangle has been set. If the Renctangle is empty, use the first children's Bound, as a rectangle.

http://www.w3.org/TR/SVG/pservers.html#RadialGradientElementFXAttribute