Also resolves tdf#92248

This extends icon name resolving so that is in addition tries to
find an icon with the "svg" extension and load that instead of
stated (mostly "png") extension. If the filename extension is
"svg" we load the icon with the SVG filter instead.

This also adds icon scaling and conversion for HiDPI or when a
dark theme is wanted. If the SVG icon is available, we render it
at a higher resolution instead of scaling

As loading of SVG icons can be computatunally expensive, a icon
disk cache was added. This saves the rendered SVG as a PNG image
into the "cache" folder. The same caching is also used for HiDPI
and dark theme converted icons so we don't always scale or
convert the icons.

In addition some style changes and DRY fixes were made to the
ImplImageTree source code.

Change-Id: I9e421395a342ffe8da9facea7ea06e5db2778b26
Reviewed-on: https://gerrit.libreoffice.org/30339Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
Tested-by: Tomaž Vajngerl <quikee@gmail.com>