Rework PlasmaSvg caching

Merged Marco Martin requested to merge work/mart/internalSvgCache into master

Move caching of svg elements and rectangles to an internal singleton in svg. the composition of the big id string is replaced by a CacheId struct, which then gets hashed to an unsigned int. This makes it slightly more efficient. the svg rects cache now has a timestamp for each svg, so is possible to update an svg without having to manually delete the cache or touch the desktop file.

Edited by Marco Martin

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