show document fonts at top of font menu so that it is easy to switch between 2 or 3 fonts within an svg document.

On opening an svg, fonts used in the svg are scanned and added to a list. this list appears in a separate space at the top of the font menu.

When text is added to an svg and a font assigned, that font is also added to the font list at the top of the font menu. When a font is no longer in use in a document (could be checked after font assign operations?), it is removed from the list.

The rational for this feature is that the current font menu necessitates constant scrolling through scores of fonts to switch between the 2 or 3 fonts that might be used in a typical document or poster. If the fonts in use in a document were listed together and accessible quickly, it would improve the usability of inkscape significantly.