Add common and useful extensions to SVG Tiny 1.2 to QtSVG

Currently Qt supports SVG Tiny 1.2 standard.

Adding a few elements from SVG 2.0 can improve the compatibility substantially without overloading the module. The most requested elements are:

• https://developer.mozilla.org/en-US/docs/Web/SVG/Element/filter
• https://developer.mozilla.org/en-US/docs/Web/SVG/Element/tspan
• https://developer.mozilla.org/en-US/docs/Web/SVG/Element/mask
• https://developer.mozilla.org/en-US/docs/Web/SVG/Element/pattern
• https://developer.mozilla.org/en-US/docs/Web/SVG/Element/marker
• https://developer.mozilla.org/en-US/docs/Web/SVG/Element/symbol
• https://developer.mozilla.org/en-US/docs/Web/SVG/Element/tref but that is deprecated.
• https://developer.mozilla.org/en-US/docs/Web/SVG/Element/clipPath (lower priority)

And the following filters:

• https://developer.mozilla.org/en-US/docs/Web/SVG/Element/feColorMatrix
• https://developer.mozilla.org/en-US/docs/Web/SVG/Element/feoffset
• https://developer.mozilla.org/en-US/docs/Web/SVG/Element/feGaussianBlur
• https://developer.mozilla.org/en-US/docs/Web/SVG/Element/fecomposite

Examples for all (but the deprecated tref) are appended here.

The task is to extend the current QSvgHandler and QSvgRenderer (and maybe QSvgGenerator) to parse and render these elements. A flag/property "StrictTiny" should be added to strictly follow the tiny 1.2 standard.