[gradient] svg round-trip (i.e. save what it loads)

export is implemented in the base class, but currently only activated
for the SVG export, not for shape export
gradient can also be referenced by style="fill:url(#x)" now
[gradient] svg: stop-(color|opacity) from style and gradientTransform

Now stop-color and stop-opacity are read by dia_svg_parse_style() not only
for our own SVG round-trip. Also included initial gradientTransform, not
passed down to DiaPattern yet. Finally some stop-color offset clipping.
svg: fix xmlGetNsProp() for "xlink:href"
svg: respect xml:space="preserve"

w/o it the parsed string is just stripped, it might be necessary to
also replace extra whitespace within the string.
Bigger drop target for integrated UI case (whole notebook space)
[gradient] Use DiaPattern for PDF import, too.
Don't leak matrix in group_destroy_shallow()
[transform] dox and fix typos
svg: improve stroke-dasharray import with odd number of values