trivial: adding SVG and high contrast icons (#3209)

* adding favicon image
- noticed that someone already contributed all my pixmaps I have been using for
  my builds (thank you, whoever you were). The favicon was left out. Adding
  that as well. It is useful from a completist perspective and have users a
  resource they can just use if they have need of a favicon for their project.

* trivial: adding SVG and high contrast icons
Purpose: for accessibility, if QT wallet is packaged (linux) correctly,
when the user switches to high contrast in their desktop environment
the high contrast icons will be used.
The packaging should land these icons in the
`/usr/share/icons/HighContrast` and `/usr/share/icons/hicolor` trees.

* trivial: HighContrast SVG is actually HighContrast now.

share/pixmaps/dash-HighContrast-scalable.svg

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