ProblemNavigationContext: fix pixel-scaled icons in problem tooltip

KIconLoader::iconPath might only reference an icon with a size smaller
than the wanted size if there is none present with a bigger size,
which can especially happen with Breeze SVG icons.

While one would hope that those icons being in SVG should still give
crisp rendering at the width & height attributes passed for the <img> tag,
in reality QTextEdit will load the image from the given path, render into
a pixmap by the given viewbox, and only scale that result to match the
given size attributes.

Instead we render the (SVG) icon to a pixmap ourself, and pass the
result as base64 encoded PNG image.