Add svg redraw of app icon following modern icon guidelines

[fpiesche]

This is a faithful vector redraw of the TFE icon following the best practices for app icon design recommended by e.g. Flathub (which implements the FreeDesktop standard) and Apple:

• The icon is on a square canvas but does not fill it completely (Flathub provide a guideline template for this)
• There is now a border around the black icon with a gradient matching the glyph, for better contrast on light and dark backgrounds alike
• The icon is saved as an optimized SVG, reducing its filesize to less than the existing .png icon

[luciusDXL]

Thanks, looks good.